I expressed myself poorly.
It's not that I have an a2 emulator that works fine and I just cannot get this game to work. It's that I have a vague memory of at some point, after reading Hackers for the 100th time, running Mystery House in an emulator successfully. The only a2 emulator I can find on my system is kegs and I can start it, I see that the virtual Mystery House disk is still there, but I have no idea how to load the game. All I see is the Apple ROM monitor program, and I have no idea how to load a disk from there.
Maybe that's how it's supposed to be and I just don't know how to proceed from there. Maybe I succeeded with another emulator that I cannot find right now.
It's certainly not the lack of a disk image, because
a) it's here: https://github.com/StewBC/mminer-apple2/releases/tag/V1.0
and
b) I'm not even at that stage, because I gave up after I couldn't repeat my success of running Mystery House.
My memories of trying to get any emulation with MESS to work pretty much match yours.
I hope you have more luck than I had.