You could but then you would still be in the MacOS desktop environment. I think he doesn’t just want to run Linux applications but instead wants the Linux desktop environment.

Desktop environments on Linux basically just draw into a full-screen X window, don't they? (And then the programs you run are children of that root window.) I wonder how hard it would be to hack up XQuartz to support that...

You wouldn't even have to hack it up. There's an option in XQuartz to make it run full-screen, at which point you can run a standard X11 DE.

The main issue with XQuartz nowadays is that it doesn't support Retina displays in windowed mode. Everything runs pixel-doubled by default.

I haven't tried XQuartz at all because I didn't think that kind of software has plenty of resource and is well maintained. Retina support is just one problem caused by lack of maintenance.

There are only two options: hack Virgil 3D or hack XQuartz. You cannot "just" run XQuartz and solve problems. And I choose hacking Virgil 3D because it should have less communication overhead and work with Wayland.

Xquartz doesn't have hidpi support (or if it has, please tell me how to enable it!)

It's a total PITA, but it can be done as long as you are ok with full screen. I use it to run PixInsight on FreeBSD and view it on macOS: https://xw.is/wiki/HiDPI_XQuartz

/edit: oh, and yeah, if you patch xrandr not to return, I suspect that the resolution change will persist on switching between macOS and the unix desktop, though I haven't tested that yet.