Actually, I'd say that these days stackage is the place to go for Haskell.

Everything flows from Hackage even if you're using Stackage.

That's a very controversial question; stack is divisive, and very opinionated (as are its authors).

stack is just software. It's software that a large portion (maybe the majority?) of the community prefers to use. Haskell is an open source project. Someone wanted to do package management in a different way, so they put in the work and wrote the code, and it turns out a lot of people like it.

I've seen some intense debates over the merits of the two package managers, but ultimately they both have their pros and cons, and are fortunately fairly compatible.

Software can be "opinionated", when it has very specific ideas about how to do things and enforces a specific workflow. That can be a good thing, if you like that workflow well enough, or a bad thing, if you don't, or if you need flexibility it doesn't provide.

Definitely. Just compare eg git, mercurial, bazaar and darcs. They can do roughly the same things these days, but have very different opinions. (And that's just their CLI. The various GUI front-ends are yet another source of opinions.)