It is certainly possible to become highly skilled at multiple spheres of magic. Kylorin mastered all 21 spheres. I get the impression that most mages focus on 2, such as entropy & dimensional, death & earth, water & air, spirit & water, or fire & chaos. (Barbatos the Lich is best known as the greatest master of necromancy, but he was also very fond of earth magic and relied heavily on summoning earth elementals. Tya Kiri was an Amurite mage who specialized in Entropy and Dimensional magic, even though she was neutral rather than evil.) It may differ from mage guild to mage guild, but I imagine that most mages are required to study at least the basics of all there spheres (except those which might be outlawed or tightly regulated in their nations, like necromancy) and then encouraged to choose one sphere as a major and another as a minor. Some may choose double or triple majors though.
While it may not be especially common, I actually think choosing to specialize in opposing spheres could be a really good choice. One might need to study more Force magic too in order to maintain a healthy emotional balance rather than become bipolar, but the opposing spheres tend to compliment each other very well. They were originally meant to be compliments rather than enemies. Before Erebus was created it was generally agreed that the best worlds were the worlds of contrast, which each god made in partnership with his direct opposite. Mastering opposite spheres would involve developing a good understanding of where each of the sphere is weakest, and help overcome those weaknesses.
Some opposing pairs would probably cause more trouble than others. Specializing in a good and evil sphere together would likely be harder than specializing in two neutral spheres. I imagine that the least conflicting opposite double major would be Mind and Meta Magic. Many angels of Oghma have fallen into the service of Mammon, and continue to use meta magic abilities in the service if mind. Being able to read a victim's memory could be of great aid in controlling his plans and actions in the future without having this influence detected.
The power of Creation that The One took away from The Twenty One was not the same as the magic of the Creation Sphere. What he removed was the power of Ex Niholo Creation, and the power to fundamentally create new basic laws of nature. The gods can however still take advantage of many loopholes that they had already created in the laws of nature, particularly my making connections between different planes where those laws are radically different. They can also continue to create things using as building blocks the mana which already exists in the world. That allows a lot, since Agares created 21 infinite plains each composed entirely of one element and stole the Gems of Creation out of the True Heaven in order to bind them to the infinite plains and ensure a way for the gods to channel infinite mana. The good gods hate him for this, but it does not stop them from taking full advantage of the power he thus granted them.
Three of the Gems of Creation were stolen from Agares by three mortals brothers. The gods are still able to draw power from the gems of Air, Water, and Death from a distance, but so can the mortals who wield them. These brothers thus have access to god-like powers. The big bad defeated at the end of Kael's long running D&D campaign was Tuoni, the man who held the Gem of Death. He intended to kill everyone in order to rule supreme over all souls as the new God of Death. His brother who held the Air gem had no such long term goal, but caused great destruction as he recklessly amused himself by hurling hurricanes around. I think he eventually became so reckless that he killed himself accidentally. The brother who held the water gem lived a long and peaceful life, enjoying time with his wife and children without feeling any need to exercise his power. He may even have forgotten that he had the gem.
(The term Creation is also sometimes used to denote that which was created. In this sense it is usually taken as a synonym for the plane of Erebus, the one world which all the 21 gods worked together to create. It seemed like it should be able to apply more broadly to include the vaults and other plains that only one or two gods worked together to create, but I don't recall it being used that way in the 'pedia.)
The Creation sphere presided over by Amathaon is not Ex Nihilo Creation. It is about originality, creativity, fertility, and emergent properties. It combines things that already existed in radically new ways to create recognizably distinct new entities. If Creation was absent from the world then mortal races would have died out, as his sphere is responsible for the conception of new life. (That goes for humans, animals, and flowering plants. Organisms that instead reproduce asexually, like vines that spread by budding, do not depend on Creation. Their growth, like the maturation of those creatures who did require Creation mana to be born initially, depends instead on the Nature sphere. Amathaon continues to look after pregnant women and young children because he is a kindly character, although technically his spheres' involvement should end just after conception and he should let Sucellus or Cernunnos take over.) His sphere is the opposite of Agares sphere, both in the sense of hope and of despair. It does not fixate on any particular goal, either to pursue it or to be disappointed when it is clearly unattainable, but remains open to taking advantage of any opportunities which may emerge from the creative chaos.
Creation is a very unpredictable sphere. It is more chaotic than the so-called Chaos sphere (which is really about Strife), but it is the sort of chaos that may lead to greater order. It has been called the most powerful of all the spheres, but also the least reliable. Most attempts at using Creation magic fail miserably. It is the hardest sphere to teach or to learn, but it comes naturally to certain talented prodigies. Amathaon is notoriously indiscriminate about those he chooses to bless. He ignores the prayers of his worshipers, and gives his bounties to the good and evil alike. He has no organized religion, but encourages those who honor him to financially support the artists whom he has gifted.
The Force sphere is really not about physical force, moving objects, or directing energy. It is about balance, compromise, and enforcing contracts.
If you could convince an enemy to make you a promise, even if he has no intention of keeping it, then force magic could overcome his free will and make him keep the oath or die trying. The effect would be much stronger if you gave him something he wanted in exchange for his word. There need not be an explicit verbal agreement either. If a force mage tells his enemy "You can have this artifact if you agree not to hurt us," then the act of taking that artifact could seal a spell making the enemy unable to attack your party. The spell would be strongest of all if the Force user sacrificed his own life in a bargain to save his friends. That could even enslave a higher level enemy to join your team and fight against those to whom he wishes to remain loyal.
If you can get enough people to agree that an imaginary line should be an impenetrable defense, then even those who disagree may find themselves crashing against an invisible wall. Kael has said that force magic is very good at producing magical barriers, which are more effective the more people they protect and the fewer they need to keep out.
If an enemy sorcerer is bending the laws of nature too much to his advantage, then a force mage could balance the scales by making the enemies spells backfire upon him. The Force sphere is good at defensive counter-spells.
Force magic can also be used to create a Geis, a combination of a blessing and a curse which could make the target far stronger yet also extremely vulnerable in the event that he violates some taboo.
Although the elemental spells would normally be altering the elements in the physical world, they do have psychological aspects as well. Fire can be used to inflame passions like anger, desire, and courage. Water can be used to calm people down. Air can be used to incite feelings of adventure, energy, and fun. Earth can be keep people focused on the work at hand. You omitted Ice, which of course can be used to summons ice elementals and snow storms. It can also slow people down, in part due to increasing an emotional sense of nostalgia, a desire to go back rather than proceed.
Body magic can also be used to inflame base emotions, especially anger. That can boost the strength of its targets, but also cause them loose control and make mistakes.
The Enchantment sphere in Erebus technically only works to enhance properties an object already has. It can make a sword far better at cutting and can make bandages better at stopping blood loss, but cannot make a sword that heals. This sphere can also increase a person's virtues, but not grant virtues that did not already exist. It can make a brave man braver, but cannot make a hero out of a coward.
Nature is mostly concerned with growth and maturation. I guess all the things you mention are acceptable uses of the magic, but making vines grow is more appropriate than talking with animals.
I guess what you wrote for life is ok, but I'd put a bit more emphasis on the last part of th quote "the pursuit of the greatest good." What is pursued does not have to actually be good in any objective sense, but the life sphere does involve a devotion to pursuing what is believed to be a greater good. It is about struggling and risking everything for a higher goal, which could be world peace or genocide.
Sending the orc into a clumsy rage does not fit with the Spirit sphere. It is quite the opposite actually, The Body sphere is good at causing rage, while the Spirit sphere is good at eliminating it. Spirit is the best for calming spells. It does not have to do with instilling any old emotion so much as with understanding emotions, and increasing empathy so that others understand others' emotions better too. It would fit better if an angry orc attacks, and the mage manages to calm him down so much that he would rather complain about his horrible childhood rather than try to hurt anyone. The mage could take on the role of a therapist, possibly convincing the orc to change sides but more likely just distracting him enough that someone else in the party can finish him off. If it was a group of orcs attacking, they would probably all turn against the one who dropped his weapon to talk about his feelings.
Chaos is largely about strife. I'm not saying that what you put is wrong, but perhaps the most appropriate way to stop an orc attack is to encourage the orcs to fight amongst themselves.
Psychologically, the Death sphere is about surrender and resignation. When there is really nothing worth fighting for anymore, it is the appropriate course of action to get out of the way. Of course, necromancy prevents this.
Entropy is certainly not a perversion of death magic, although it could be argued that necromancy is closer to entropy than to the proper meaning of death. Hope is about having a attainable certain goal to desire and pursue. Entropy is about the despair caused by false hope, where one remains obsessed with a goal despite not believing that there is any way to work towards its accomplishment. That creates a sense of pointlessness and leads to the neglect of other pursuits which may be more worthwhile.
