the most Youtube is gonna do is take down both videos, they really don't want to be involved unless its something serious, and their forced by law to do so....
they wont get in the middle of a dispute, or weigh sides....back a while someone wanted my name, and they kept pushing the issue, all YT would comment if this guy pushed the issue, they would remove both our names, and it would not be allowed to be used by anyone else...we could signup again, but under a new name...
they may knock this person out, just like ebay, but that person can get a whole new IP address and come back again and do the same thing......
the only recourse for something like this is to hire your own lawyer, and seek restitution....which can cost money and time....and you may still end up with nothing...just the way it is....
best thing, if at all possible, is to post your screen name in the actual video, its the only proof that it belonged to yuo....