Problem is, insurance work is big money for a body shop, so the private work tends to get pushed to one side. Unless you have a written contract with the bodyshop with regards to time scales and monies to be paid etc then I wouldn't have thought there is a great deal you can do. I would expect to pay for the work that has been done so far, minus an agreed amount for the excessive piss taking.

Don't forget as well, a new body shop will probably want to start all the prep again, as 90% of a good paint job is in the prep work. I doubt they would want to paint on top of someone else's prep work, as it is their reputation on the line if it all goes tits up.