You sound like you know a lot more than me in this area, but seems logical to me, despite it not being notarized, there is proof the company was paying the stated amount in the contract and they had been working together over that period, so I'd think that should hold some weight too. /shrug
Yes, there would be a paper trail, assuming the client never paid cash.
The difficult part is getting the proof if Kosher1 didn't take copies of the check. To authenticate the transaction was happening he would have to have a copy of the checks or the clients bank records. Paypal records wouldn't be good enough, not for the rules of evidence in court. It would have to be from the clients bank records.
Getting them is an entirely different ball game. You would have to ask for them during the discovery process or subpena them.
Basically, the court process could take more than a year and may end up with nothing.
The other question you have to ask, are they worth suing. Meaning, do they have money. You can take people to court all you want, but if they don't have money, they won't pay.