Hindsight is always 20/20 but I'd have sourced a new engine and had it fitted. This is a complete nightmare. Generally speaking, non-franchised independant garages (or back street garages) have a great deal more experience carrying out full engine rebuilds and engine swaps. A lot of so called "technicians" at the main
dealers are nothing more than glorified Kwik Fit fitters. Fine for doing tyres, brakes, exhausts and
servicing but I wouldn't trust them to do anything more serious. A cambelt change is the most I'd allow them to carry out on my car and that'd only be if I didn't have time to do it myself and didn't have a trusted local
garage that I could get to do the work.
I suppose you're stuck with them now as they'll need to put right what they've done wrong but maybe something to think about in the future.