That is the thing, for the main couple, are they really "happy"? Seeing their chemistry, I mean I would think yes... but at the same time they're kind of locked in this codependent relationship for years to come. And the pink haired guy doesn't have infinite lives to give. I think this alludes that eventually, it will end as well.
I see what you mean, it is a kinda tricky question XD
But to me they have a “happy ending” as the story end and what happen after... well who knows?
Because Death is part of life, especially theirs, so If they enjoyed their time together even If the Uke died definitely it will still be worth it to me... And they’ll have a looong time till then... ╮( ̄▽ ̄)╭
As for the codependency relationship... well everything is a question of balance and point of view so it’s difficult to say if they will end up feeling “trap” in it eventually...
Ok to much philosophical sense stuff for today XD i’ll Stop here or I never will

Liiteray no one.
I can somehow accept that.
If I think too hard about it, it’ll consume me tbh. I agree that some parts could have had better background development, but i think the story was worth reading. Even the fucked up ending.