Agree with Phil and is what i said earlier, if you knew was going to cost $800 to get going???? plus the rest for the ecu why not buy a name branded ecu and be done with it?? [x2x]
Sorry but the whole point of it is for DIY to save the cost of setting up etc, and im sure these guys dont appreciate wasting 7 weeks on it. Had you even tested the ecu was working before you gave to them?? might be a start it could be a dodgey one and no wonder they haven't achieved anything. All those little resistors and pieces can cause a lot of problems and many hours of thinking, which should be done in the shed at night drinking beer swearing at the dog, not by them trying to run a business.
He made a mistake by saying he could complete it nobodys perfect, but a main chip that has been overheated when solidered onto the board he will have no idea about, nor anyone else that hasnt had expirence with them. These things do happen as i found out somethings the long and hard way myself.
About legal action, you haven't had your car done to it what they said fair enough, dont pay $800. I would offer him half as an appreciation of the time and effort he has put into it. If he is charging you more then obviously he has spent a lot more time then he estimated on it. Remember they could have said no and done 6 other cars in the meantime, think of his losses too and finish it at that.
Plus if you get sheetty at him whos to say he wont undo some work that has been done which is what i'd be doing in his case not to mention some destruction to your car.