I was able to safely remove the spring by compressing three more coils with the cheap rental (added to the five already somewhat compressed by the center bolt compressor). That's the good news (especially the "safe" part).
The bad news for me is that these springs don't seem to be the right ones for this car. They are wider in diameter than the shim and don't fit in the upper cup at all (see photo below). Obviously, it would have been better if I had put the shim on before trying to insert the spring, but I didn't have enough space to do that, and it never occurred to me to check that it would fit. In retrospect, there are several reasons it should have occurred to me, but we don't need to go into that.
Thanks for the help, everybody. Maybe this will be a lesson to folks in the future.