After writing this post I did start to look up 1/2" nuts and it seems like those are available so it may need to be the route I go down. I did actually find some similar nuts to those linked above with the 60 degree taper, so thanks guys.
However there's still another problem, the standard 46mm Volvo studs are too short for the front wheels, I only get about 5-6 turns of engagement with my old nuts, however the rears are OK. So I would need to source longer 1/2" x 20 studs (in the range of 56mm-61mm total length) for the front but finding one with the right knurl diameter is still an issue - the 15.7mm ones I found were the closest fit. There are some others I came across but they are really long - 68mm and longer. Kaplhenke/BNE have their extended studs but they're like 80mm.
Regarding the 16mm vs 15.7mm, I have a Volvo stud at home that I measured with vernier calipers and it was 16mm exactly, the 15.7mm ones are just according to the measurement on the website, so no, not a consistent measurement.