This is very speculative, but along the things they’ve learned for VR they‘re mentioning APUs and wireless streaming. This might hint towards a standalone device which can also be connected to a PC.

It‘s possible to use VR headsets with the Steam Deck. It‘s just far too weak for a pleasant experience.

It‘s worth noting though, that, according to this interview (37:30), Valve is probably working on a new VR HMD which will make use of the things they‘ve learned from developing the Deck. So I‘d guess there‘s a standalone headset coming from them at some point in the future.

I‘d be really surprised if Apple tried that.

They have to know that it violates the DMA. And the penalty for violating it can be up to 10% of their yearly worldwide revenue (not earnings!) for the first violation and up to 20% for repeated violations. I don‘t think they‘d risk that, especially as the EU really isn’t known for its leniency when someone intentionally breaks their rules.