You have accurately summarised what I had discovered.

I'll stay with the brass plug I have installed and see how it goes. The machining in the block didn't seem to have a groove at the base so I can't see how the "flat" frost plug would be any more secure than the "cup" one assuming that they both fit with similar interference. The flat one relies on it being expanded, whereas the brass cup relies on it being oversize and springing in as it is hammered in. I'll keep a close eye on the temperature gauge till I am comfortable that it is there to stay. I'll also carry a spare plug in case it does blow out. At a pinch I could wedge something in between the plug and the alternator bracket to hold the plug in to get me home.

The problem in New Zealand is that there are no local suppliers for old Volvo parts. I might think about getting a few the next time I place an order from Sweden.