Basium's death would presumably just send his soul back to the Netherworld. Since he is of the sphere of Life, which is about willing self sacrifice as well as miraculously fast and complete healing, I imagine that he could regenerate faster than most others. Since his job before his fall was guarding all of the passageways out of the Netherworld, he could likely escape the Netherworld easily to fight again.
It would make more sense to try to capture him. The only sure ways we know to capture souls so far are using his own weapon (which traps evil spirits within), or using the Netherblade (which binds souls to the Netherworld).
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We don't know much about Tali in the Godswar. Both he and his opposite, Kilmorph, were of neutral alignment. The neutral gods were sort of technically on the same side as the good gods, but their concern was maintaining the natural world rather than fighting for the souls of men. They typically did not fight, except to defend their own territory from invasion. It is not clear whether anyone actually managed to invade the strongholds of Sucellus or Danalin. We know that some of Kilmorph's territory was invaded by demons and laid waste, and that she fought back. Tali's disposition is very different than Kilmorph's, so it seems likely that he would not just stay put to defend a certain area. He is adventurous and playful. I can easily imagine him joining in some battles from time to time, or pulling some pranks that makes it harder for others to fight. He is not a fan of planning though, and does not typically except his actions to have significant consequences. He would not have had any real strategy or long military campaigns. His actions may have been moderated somewhat by Danalin, whom he considered his best friend.
At Cassiel's request, Dadga got all of the neutral gods to band together to insist on enforcing a ceasefire in order to allow the negotiations over The Compact to proceed. Danalin and Sucellus were the most important members of this alliance, since they had stayed out of the war so much that they had yet to suffer any losses. Tali was definitely part of this alliance, but I don't know what he did exactly.
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Erebus is considered an Infinite Plane rather than a Planet. It has not been revealed what lies beyond.
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The battle lines that later determined who would and who would not fall originally emerged in relation to what ought to be done about Gabella.
Esus, as the Angel of Trust, believed that the gods ought to trust Gabella to make decisions for herself.
He also thought that The One should have trusted him and his brethren enough to let them keep the power of ex nihilo creation.
Mulcarn, as an Angel of Nostalgia, thought that Gabella's refusal to mate with Nemed was a good thing. He dislikes change, and so had come to the view that creating humans at all was a bad idea. He opposed creating Anesidora to be the progenitor of a mortal race. Since Gabella was already around, he didn't feel like changing that either.
He of course also wanted to go back to the time when he had the power of ex nihilo creation.
Aeron wanted Gabella for himself. The angel of the body was extremely proud of his work in forming her, and lusted after her sexually. He was jealous of Nemed, but not wiling to give up any of his own strength in order to become the father of humanity. He wanted Gabella to bear him children who would not only inherit immortality, but would inherit the full power of a god. He intended these titans to overrun Creation and force all lesser being into submission to their father. Aeron was the first god to become not merely a bit rebellious, but truly evil. It was to prevent Aeron from raping Gabella that The One last entered Creation.
Aeron of course also wanted to keep the incomparable strength allowed only by the power of ex nihilo creation.
Camulos has always been an angel of anarchy. This now manifests as strife, where every individual exercises his freedom with disregard the harm they do to others. However, it was originally about the harmony enjoyed when no one wishes to compel the will of others. While Junil insisted that Gabella must be destroyed unless she at one submits to fulfill the duties the gods assigned her, Camulos did not believe that the gods had any such right to assign her any duties. Camulos was committed to defending Gabella's free will, and would consent to destroying her only if she personally requested it.
Camulos also did not like The One infringing on the gods' wills by removing their power of ex nihilo creation.
(Gabella was very confused and overwhelmed by her sudden existence, but she did not become suicidal until hundreds if not thousands more years had passed. She had also not yet taken to calling herself Os-Gabella, or Gabella the Unbound.)
Agares sided with Gabella because he believed that her rebellion was a reflection of his own, and believed that The One would destroy him just as his brethren were planning to destroy her. He hoped that she could be allowed freedom just as he hoped to have his own freedom, but in both cases he despaired because this hope seemed so unlikely. He became the leader of the evil gods because his hope and fear had led him to create infinite plains of pure elements back before the power of ex nihilo creation was removed. Channeling these elements through the power of the Gems of Creation (which Ceridwen had helped him smuggle out of the True Heaven) allowed him (and those with whom he shared the power) the ability to keep creating as if The One had not removed the ability. (All of the gods have taken advantage of the Gems of Creation, even those who condemn him for making them.)
I would say that Mammon fell not simply because he did not want to die, but because fear of the unknown (which seemed like death) caused his desire to control the future to overwhelm his ability to predict the future objectively.
No one really understands Ceridwen well enough to know why she fell, or why she does anything. She has always been mysterious, even to the other gods. It does not seem like she initially wanted to destroy creation, but no one knows what she did want.
