I'd be getting my stuff out of that shop, asap. If they are fiddling the books trying to make up for a shortfall it is very possible they have royally screwed somebody else. You can see and touch your downpayment - a rolling chassis at the least. If another person sues and takes them to court then your stuff might get bound up in litigation, if you can't prove it is yours then it might get sold by the courts.
It is all conjecture at this point, I really really hope that it don't come to pass and hate to hear of things like this. But, they have given you good reason to worry, you aren't satisfied with the progress, the quality of what work that is done is in question and they have lied to you.
I know what I would do.