But the OP would still have a verified Steam account so that really doesn't help him. There really is no way to run either of these two HMD without forming a connection with the corporate bodies behind them.
Sorry, Koen, but you'll need to change your stance before you can use either of these two VR systems.
Yes, probably. At least Steam is sort of the best of all DRM Clients. Koen could just create a garbage e-mail account and register it to steam, done - no big deal.
Valve is in general more open than Oculus, technology wise. With the Oculus store you get things like this: If you bought Dirt Rally from the Oculus store, which has a higher pricing in general, you can play the game in VR only. The Steam version can be run in either flat or VR.
Well, I'll keep an eye on the market & on OSVR and see what comes up next. Or perhaps consider a beamer instead.
Since Oculus and SteamVR are established in the market, it is rather unlikely, that a competitor uses a different API. And if it did, released games had to be patched in order to work with it.
Edit: Btw. a VR headset is a supplement to a screen or beamer, it does not replace it. So be mindful with your choice.