The unhappiness from the girl is the same as whipping (slavery) unhappiness. This means that 1 :angry: goes away every 10 turns. For saving the girl, you get 5 :angry: - so 5 :angry: till turn 10, 4 till 20, 3 till 30, 20 till 40, and 1 till 50.
As for the magic. As long as you have at least 1 of a mana type, any unit with channeling 1 can take the level 1 spell of that sphere. Generally, that means adepts/wizards/archmages, but there are a few exceptions (mostly the exceptions are found in the modmods). An important thing to remember though is that some of the spells wear off, like blur (shadow 1), and dance of blades, meaning that if you want to use them, you need to accompany your army with an adept or two to cast these spells. As for the walls, that and most other mage buildings (all of them in base FFH IIRC) only last for as long as the caster is in the city. That means that wall of stone is only there as long as you have an adept with earth 1 in the city. Same for any other mage building. For the promotions that you mentioned, no, there isn't any reason not to give them those promotions. If you have the mana for it. The problem comes in that there are 17 different mana nodes that you can build (18, but you can't build ice nodes), and you have to choose which mana types to build. So do you give them courage? Or do you give them enchanted blade? Or do you get spring to turn all your desert to plains? That is the real choice with adepts. The other thing to remember is what mage spells do you want? Do you want flaming arrows? Fireball? Summon Specter?
The other important things about magic are as follows. With just 1 of the mana type, you can choose that promotion. With 2, all your arcane units (adepts), gain the level 1 spell of that sphere for free. With 3, all your arcane units with channeling 2 gain that promotion for free (so if you have 3 earth mana, and you build an adept, he automatically gets wall of stone. If you were to loose one of the earth mana, when he upgrades to a mage he gets nothing. But if you were to still have 3 earth mana, he would then get stoneskin for free. Similarly, if you have 0 earth mana when you make the adept, but get 3 before upgrading him to a mage, your mage would start with earth 1 and earth 2). With 4, all your arcane units with channeling 3 gain the level 3 spell for free. The second important thing to remember is mana affinity. Basically, affinity means that for every mana you have, the unit gains one strength. A few examples of this are specters (death 2 summons) which have +1 death affinity, Chalid (empyrian hero) who has +2 sun affinity, druids who have +1 nature affinity (except dwarven druids, which have +1 earth affinity).
So basically you have a few choices. What mana's do you want? What spells do you want? Are there any affinity's that you're going for? Do you want your arcane units to get any free spells? This is why everyone says that magic is complicated.
-Colin
points, although of course if you choose to build any of them that will mean a permanent dilution of your 
