<<Is it your belief that when Doomsday is defeated by a certain means, that when he returns he will be totally immune to the means by which he was defeated, or resistant to a certain degree?>>
i tend to think the latter. i think the idea is he can evolve beyond a certain enemy - ie - (and this is all speculation) he could be killed bby an energy blast from x, but when he returns, it would require a blast GREATER than x's to kill him. dd has been killed by imperiex and weathered the omega effect, so to me it seems reasonable to say thanos or ss's blasts wouldn't be enough to kill him. his strength is also far beyond supes'.
as far as whether supes was weakned when he was killed by gog -- i don't think so. jinzin actually posted the battle scans somewhere. maybe he remembers where. in any event, he could answer the question better than i can because he actually has the books and has good knowledge of the arc itself.
when talking about dd, much is left to speculate. in his ultimate form though, he IS sentient, brilliant i think, and has not been defeated. there has also benn nothing yet to show there is a limit to his ability to evolve. perhaps when his tale is all told, i'll be proven wrong or a limit will be set. as of now, there is no reason to think the ultimate wouldn't be a match for thanos.