It's either OpenSUSE or Ubuntu based.

Most likely ubuntu as that seems to be their defacto test platform. If you download steam from their site it comes packaged in a .deb format. They have pretty significant investments in that stack, and even list it as their favorite flavor:

"Not running Linux yet? Ubuntu is our favorite version of Linux."

(at the bottom of this page: http://store.steampowered.com/about/)

I hope not. Going Ubuntu will mean (long term) Mir instead of Wayland, which will suck for other distributions and projects.

Likely, but of course just a speculation at this point.

Likely, but do you have a source for that?

Does not say.