Ok, it took me a few days to get the time to take this all apart, but I do have a spacer behind the cassette. It does not appear that the cassette cogs are bent either.

The "wobble" is definitely something with the freehub, of which I will not be attempting removal for inspection. It's hard to see without putting cogs on, but I put the smallest cogs on, then the larger ones, put on the lockring, slowly spun it by hand, and then it was the smaller cogs that seemed the most "bent" and the larger ones less so or not at all. I should add that when I grip the cassette I can feel a tiny bit of lateral play.

What could this be? Something crushed or bent in the freehub or axle? Something worse? It it possible a new freehub would alleviate this problem? I realize this is difficult to know without seeing it, but you guys have experience every kind of road wheel weirdness I'm sure.

Besides the visual evidence as to something wrong, it's not possible for the chain to stay in the largest cog - it drops back to the 2nd unless you hold the shifter in place, and also makes a little noise in the 2nd cog from chainrub caused by the wobble I guess. Otherwise seems to be rideable and not about to catastrophically fail.

I can't get in touch with the seller until tomorrow, so if you have any suggestions as to what this could be and what a reasonable solution would be, that would be fantastic - I really want to keep the bike, but at a premium price I don't want to keep any busted stuff either.