The way the technician explained the problem to me was exhaust from the combustion chamber was making its way into the radiator fluid (we could clearly see the path across the head gasket too) and this was over-pressurizing the coolant system causing the flow to be much slower or completely stopped (almost like having two pumps pumping in opposite directions).

I never tried running the VX with the radiator cap off so I don't know what to tell you there.

A decent garage should be able to determine if the head bolt is broken off in an hour or two, so check their hourly rates. Tell them not to loosen any of the head bolts, just look for a broken one. Once loosened, you have to replace them with new bolts. You might be able to get away with just replacing the one bolt, assuming they can get the broken piece out.