6. Do they believe that suffering is necessary?

Pretty firm no. He also doesn't think he's suffered all that much and gets mad when it happens to other people. Everyone in his life wants to shake him about this.

19. How do they react to the concept of eternity?

His perception of time is already really distorted, eternity might as well happen. ¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯

(The actual answer is that he's very much a One Day At A Time kinda guy. He'd handle eternity really well.)

21. How would they react if they learned that some of their memories were not as they remembered them?

lol I was going to say that this is just a normal Wednesday for him, but upon reflection I think this would and does actually fuck him up pretty bad. His usual issue is memory loss. When the issue becomes “too much memory” or "this couldn't have happened this way,” I think his brain hits the “Actually Yes It Did” button really hard and this approach has yet to cause problems for him.

It's happened in-canon (of course it has) where someone will be like, “Yeah, the twenty years that we spent in the Nightmare Dimension weren't real,” and Mal might externally nod and agree, but internally he's rejected that idea and has decided it's all real to him, even if the logic of it doesn't work. Memory hoarder of a lad.

36. Would they behave differently if they knew they were always being observed (but not judged) by a higher power?

THIS is actually a normal Wednesday for him lol. Many such entities.