We both know he can sign a message to legally prove he owns darthknight. Digital signatures are well established and I would personally avoid lawyers who are unfamiliar with finance and blockchain for such cases.
The issue is the frozen account. If the account wasn't frozen because of 22.8888 and the witnesses deciding to run this code, the problem would be solved.
Who owns bittix is a red herring argument unless you have evidence.
You suspect he is bluffing on the compensation due to bittix.
If you are wrong, witnesses behind 22.8888 may be additionally liable.
If you are right, he isn't lying in court and witnesses still froze his Steem.
Given the circumstances it is more of a dangerous gamble for you, not him.