Speaking as someone who agrees that the story of this game is a shambles, I do honestly think that several of the points you raise are ones that don't really matter all that much and would be totally acceptable if the rest of the story were better. For instance -
1. He figured stuff out and got into arms trading at some point. Same goes with
2. How he found the info is again not really the important part. It's part of the background and nothing especially interesting or meaningful to the plot was involved.
3. How they got the crown and why the archdevil who had it doesn't care is a worthwhile question, especially the part about Mephistophelese because that's a very reasonable, very interesting wrinkle. Like Raphael could have been working for M and trying to get the crown for himself. That's an improvement on the current plot that's both more complex but also creates a clear progression of events and motivations. As for why they share the crown for themselves? That's actually pretty easy to ansswer; they were told to do so by their respective gods, that seems like the obvious answer.
4. Yeah this is a little strained. The oath he gives is supposedly magically binding so it's actually not the craziest thing. Apparently he made Orin make a similar oath and she can't directly kill him as a result, so it's clearly pretty powerful.
5. Presumably this happened through the intervention of their respective semi-divine patrons. Elder Brains are powerful, but I don't have trouble believe that 3 semi-divine beings can pool enough power to overcome one long enough to put a crown on it. That it was a very risky plan doesn't really matter though. It worked, and we're dealing with the consequences. That's not even a plot issue, it's just an observation. For all we know they had to muster large amounts of resources and engage in a borderline war. What bothers me more is that this colony was seemingly the one under Moonrise Towers, that's been under the area all through the stuff with Kethric when he was a paladin of Selune, when he went to war, the shadowcurse, all of that and seemingly had nothing to do with any of it.
6. Yeah this whole thing with the Emperor isweird and a continuity mess. To be fair he didn't "casually" break free, it was a freak occurence, one that the game gives us another example of before we learn about the emperor. So him breaking free isn't the biggest issue. But how he ended up in the prism and why is all very strange and ill-defined.
7. Actually, the more I think about it the more sense it makes. He may have ultimately lost, but he still led a protracted war that required the union of two different factions to overcome him. Plus before that he successuly subverted the region into the control of Shar, captured a powerful celestial being and generally was a formidable presence. Plus if you read some of the texts around Moonrise Tower you see he came up with very effective strategies to bring various factions under the cult's control. He's clearly a very good general, you can be good at something and still lose. Regarding his loyalty, that actually makes perfect sense too. He is consistently motivated by Isobelle throughout everythingin his story; he turned from Selune to Shar because he was overcome with grief for her death and wanted Shar to make him forget. So of course Myrkul would look at that and rightly think that if he were to actually give Isobelle back to him, Kethric's loyalty would be assured. It's a very safe bet for Myrkul actually. As for the other stuff about resurection, I again put that under stuff that wouldn't bother me if the rest of the story were better.
8. Fylimar explained quite succinctly why Orin's actions were in-character. Dumb, but entirely in-character.
9. Yeah, that's weird and dumb.
