So, because I am in fact kind of wary of your oath and don't entirely trust anyone on this plane yet and your actions are cursed to end horribly, I'm going to be handling most of the development for the god-killing spell myself. What I have thoughts for you on are auxiliary priorities. Divination is the big one - just in case Angband or Melkor's presence interfere with magic somehow, divination is the only really reliable way to test that at a distance. Also, solid divinations will be a huge help for targeting. Better teleportation will help logistics immensely, and is tactically useful. General research into targeting spells through divinations, because I don't think we want to have to get within line-of-sight to Melkor to destroy him. There are some spells in the Enhancement sphere, which can be worked into magic items or made permanent, which will boost some aspects of intelligence. Doesn't really speed up spell development much, but it can help with casting more powerful or complicated spells. I still cannot for the life of me get healing down myself, and yours is more sustainable, but I still think having both will help immensely.
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Time and Fate are still super dangerous and hard to do any experimental research in but Fate might be worth it. I don't know if it's possible or how it would work but it seems like Fate might be able to do something about oaths or the Doom - it can break curses in general, but those are curses that run off a similar magic system and aren't laid by deities. Having all your efforts not fated to end in disaster seems like it might be a very worthwhile step to take. Or the Doom might cause your attempts to subvert it to end in tragedy. I could look into it myself but that would take a lot of time away from my other project.