what to do?
You're overthinking it. Nobody cares about those garbage negs. Nobody that matters I mean. Forget it.
I would advise you not to leave a retaliatory feedback just for that. If you were not on DT, it might be arguably OK but my feeling is that you have to pull back a lot when you're on DT.
Correct, for a non DT it's ok to do whatever they want (use/abuse of anything), nobody counts but once someone become a DT then they are basically a role model/an example for the forum.
I can't tell if you're being sarcastic but I think that's wrong. The trust system should be used responsibly at any level.