OK, the demineralized water may have done the trick. Worked it out with the switches upside down. The problematic Caps Lock started becoming rubbery at some point (indicating that there still is something in there) but now that it's dried, it's only somewhat rough. No sticking any more, and it'll actuate from the side.
Maybe I'll do another cleanup one of these days, but for now this does the job.
All, in all, for cleaning up spills on Cherry G80 boards I'd recommend: Dry cleanup only. If the stuff didn't get into the switches by itself, i.e. they work fine, best leave things alone. Don't learn the hard way like I did.