What happens if the rear wheel bearings are seated "so well" that both bearings and the spacer will only turn as one unit now?
I believe that is what happens normally, after you tighten the nut. That is why the spacer is in there, to tighten up between the bearings. Otherwise, you could have left it like you first did, without the spacer! LOL

You are good to go. Anyway, I know of no one who has one side of the wheel turning at a different speed than the other side!

MP