tbh any "didn't use raise on his brother" interpretation would require what amounts to total character assassination of clive, there is no universe in which the clive we know doesn't do that for his brother upon learning the raise spell, regardless of the consequences to himself, so i definitely think he saved joshua before remaking the world no matter what.
then to me clive's dialogue after the fact overtly states that even the mythos vessel wasn't actually designed to survive the casting of the spell itself; this lines up with ultima earlier talking about keeping the "used up husk" of his previous vessel, the one you fight. and it makes sense too, ultima doesn't care whether the vessel survives, he just needs it to complete the spell and then discards it. so giving mythos enough aether to do more than that is pointless.
this is satisfying storytelling; there is a cost to that much power, so much so that ultima and his civilization entirely discarded their material existence, then instigated a multi-thousand-year-plan to create new vessels (more than once, by implication) in order to be able to use it. if clive suddenly gets off scott free, what was all the fuss about? ultima could have just tanked it himself, would've lost a hand for it.
so no, in that moment on the beach, i think clive knows he's sacrificed himself for his brother and the world, and is at peace, his only remaining thoughts being of jill.
this is thematically clean, satisfying narratively and preserves the dramatic emotion of the finale without neutering it by going "sike! he's fine".
so i think it's all pretty much overt: joshua lives, clive dies, joshua wrote the book. that's the best ending imo.
really don't get the who wrote the book stuff as needing to be ambiguous, a lot of the evidence is like "clive said he'd write a book" but like... not only does that interpretation assassinate his character (chose to not save his brother when he could, saved himself, but then wrote a book in his brother's name to make up for it? or did save his brother but still wrote a book under his name?), ruin the drama of the ending (sike! either clive is fine or both joshua and clive are fine! everyone rejoice!), and everything else that happened... but on a basic level people say they'll do a lot of stuff before they die, it doesn't mean they get to. he said he'd write a book if he came back from the fight with ultima. he didn't come back, so he didn't write the book. it's a very weird piece of discussion to me, almost mass fan psychosis level with how many people believe it lol.