never let your V5 out of your hands, only pass on the new owner slip, and keep a copy of the V5 before submitting it to DVLa. Doesn't matter if the new owner details are false as you've then carried out your obligations as previous owner and your protected. Unfortunately as you let the log book go with the bike, you've dropped yourself deep in it, there's no solicitor or defence that can get you out of that, in my opinion and you'll be throwing good money after bad.
Trouble is, from what your saying, you might also now get hit with more penalties from not notifying the change of owner on top of the parking charge you currently are disputing.
Your immediate action, in my opinion would be to do whatever you can to protect yourself from other offences this guy may commit in the future or your nightmare may become even worse ! driving off without paying for fuel, speeding ..... etc etc