Just because something is accessible publicly doesn't mean it's suddenly legal to copy it, same as it isn't OK to go into someone's house just because the door was open. Unless you're police for some weird reasons.

No, for the resulting open drivers to not be legally dubious the spec can only be obtained by doing a clean-room reverse engineering.

Legally dubious in what sense? Leaking it might break trade secret protection, but afaik once it's public, it loses that protection, and the only one liable is the leaker. As far as I know, software per se is still not patentable even in the US since the actual source code is abstract mathematics, so it should be fine to publish the source (source code is fundamentally a detailed description of an algorithm, not a system implementing it), and there's effectively no way to stop an end-user from compiling and loading that source themselves. You could also distribute it from a more reasonable country like e.g. VLC does.

Well if you can download the source and compile it, I don't think it being legal really matters, just host it in a country that doesn't care.

The problem isn’t that people don’t know how to do it.

So what, just the trademark issue for "hdmi 2.1"?

Call it a imdh driver then, nobody cares as long as it works.