So, is it worth for a red trust from DT?
No, I don't think so. Posting only English board after changed password is not enough to red tag. Most likely account has sold since one one complained about hack. There isn't enough evidence to tag him. But if user post on others local language except Russian then it's worth of tag. Chamged password gap almost a month. Someone could change pass even every week he shouldn't tag (imo). Need more proof to tag him.
but you can ask to sign the address.
In general, DT Member himself decides
Will it right to ask anyone for sign message due to changed password ? May be he didn't stake address.