I mean in terms of developer interest, I haven't seen anybody working towards getting those patches (or similar things) merged. It would be easiest to get this working in a rootless server (like XWayland) first and then go backwards from there. In the case of compositing on multiple monitors, scaling isn't independent of DPI settings. They are the same problem, because at some point you will get an app that is mismatched and you'll have to scale it along with its input coordinates.

The physical measurements could be used to inaccurately guess defaults, some Wayland servers might do that, but it's not preferred. The X server doesn't "misreport" DPI in the sense that it's purposefully sending a wrong value, there's a process that it does (I may not be remembering this exactly) where it tries to read from the EDID, which could be wrong values. If the values are there but wrong those will just get quietly passed through, if those are zero then it then tries to fill in measurements based on a default DPI. Random X clients with access to RandR can also change the measurements stored in the server for whatever reason -- basically there is no possible way for this value to always be correct if you just want a number that tells you what DPI to target.

You're right this could be fixed, just like every bug could technically be fixed if given enough attention, but somebody needs to take the time to backport the Wayland method to X, which I don't have high hopes for it happening any time soon. The main thing you're missing in the server, as I said, is scaling of input coordinates. The compositor or the toolkit can't do this, it has to happen in the server, because those events are sent directly from the server to the client. It's probably a better approach to get that working, and then put a prop on the root window, and then hack kwin/mutter/whatever to use that, but I don't think anyone has tried to implement it that way yet.