You are free to do as you like with xna on pc.
So much like any other API then, and still much less than a proper 3D engine. a) It runs on Vista and XP.
And you know this because you've tried it on every conceivable Vista configuration? In any case Microsoft QA guys have a configuration that it doesn't work on, and that's what I'm going by - not whether it works on your PC or not. b) It is great as i know of a few small firms which exactly went through this.
Name one. I know for a fact that neither IO Interactive nor Deadline Games went through XNA.The idea that XNA offers any advantage over any other API, when approaching publishers is ridiculous. If you know of companies who got a break, good for them - but chances are they would have gotten it regardless of using XNA. Sure if by "publisher" you mean "Microsoft Casual Games" and by "console" you mean "XBox360" then maybe. But I seriously doubt that the real or imagined advantage is worth the cost of the exclusion of every other publisher or platform in the world.
c) You don't need to know.
Well I'm certainly not going to just take your word for it, so I guess I do need to know. d) I can utalize the hardware fine via XACT, via shaders, ...
So you concede that you cannot access the hardware using XNA? Maybe rereading one of the above comments helps.
The fact is that you cannot share your XNA game with other XBox360 owners. Seeing how Live Arcade is doing much better than (most) commercial XBox360 games, it's somewhat questionable that you ever will be able to. Using your own definition of sharing doesn't change this.