On paper, and tbh,seeing as you haven't even gone to the effort of finding out how much damage or what injuries you left this guy with out of concern for him or yourself, letters from german courts seem to have been ignored and you seem to have just laughed it off. I don't like to think of...
Yep, lawyer the shit out of this. The "victim" has brain damage so you'll need to shield yourself against a civil case too. In all seriousness, good luck, hope you get away with it.
Oh, regarding your witness: The Hun hate septics, regarding them as ignorant, sex crazed bullies with no manners, and a rep for not being able to hold their liquor.
One of the most common things drunk blokes say when they have got all pissed up and lamp someone is the ol' "saving a lady" scenario. It's reasonably clever in that if you are guilty you can give the impression you did this for an "honorable" reason. It's flawed because it just smacks of...
So, you drank over 10 litres of beer, claim to have knocked a man out because you believed he was sexually assaulting someone, but she is in the US so will unlikely back you up, you are arrested, you pay bail and have a giggle about it with the German Police who laughingly tell you to forget it...