You are in a Catch-22, because good faith belief/innocence is an obvious defense to the crimes. The easiest way to prove who wrote the check would be to obtain hand writing samples, but that costs money, as would hiring a new lawyer. Plus if you drop out of PTI, then you risk the possibility of jail at trial, whereas now you guaranty a dismissal upon completion. Tough choices.
Ouch!!! I know the tree business is very tough. If they bounce you out of the PTI program for non payment your case will go back before the judge assigned to your case and the State Attorney will prosecute you. It sounds like you have defenses. Did you have an attorney? If you did, you may want to contact him again. I know that it will cost you more than $2800.00 to prepare a defense and go back to court. You need to consider the costs. It sounds like a "no win" situation. Call me at 407-831-3434 if you want to my help.
In my opinion it is Mike that owes Charles the $2800, not you. I agree with the suggestion of a handwriting analysis done. They can be expensive however if you cannot afford to get that done an attorney can file a Motion for determination of partial indigency for due process costs for you. If that motion is granted then you can get the analysis and the state will have to pay for it. You also want to make sure that any proof you have that the trees were cut is filed in reciprocal discovery.