1 needs more context. It's possibly 'what we/they will play next'. I don't think it's 'what', as that might make it a question.
2 doesn't make sense. Again, what's the context?
3 There is no 'it' in the lyrics I read, and it doesn't make sense with 'it'. It's referring to 'her'. You see her in your dreams, but of course you can't touch her or keep her.
4 Yes, sort of, but more 'probably very soon'.
5a I am in it = I am part of their/her story.
5b I think the whole sentence is referring to the writer's conception. They are referring to the instant (millisecond) when it happened.
I guess most conceptions happen in the dark? Or it could be suggesting that it was a dark/bad moment for them. It's not clear.
'years ago' suggests the writer isn't young.