So I'm assuming that means that Valve programs this in a way that dictates the hardware, instead of having an abstraction layer that accepts player position, viewpoint and hand inputs regardless of the hardware?

Is that something you have a source to confirm?

I would like to think that if Quest is going to release a way to hook up to the PC to play SteamVR, they'll be sending all that positional data from the headset tracking to the PC through the link.

I have a Vive, just been cagey about updating. The Quest looks like the kind of casual experience that I wanted after having a Vive since launch.