First attempt at mechanics, went pretty badly...

One question though, I didn't think it was possible to overtighten a lockring, I thought the point is for it to be as tight as possible (at least from the threads I read here). If so then clearly it was a bit fucked too, I just took it over the edge.

it's possible to overtighten anything. it would normally be fine (i.e. you haven't been subjecting the poor thing to being loose for so long) just tightening up by hand.

you can always buy a rear brake.