I would assume 63.4mm, pretty much what all Volvo's are now, so the same as the P3 cars. You would need to look into whether that +45.5mm offset would work on a 20" wheel on these cars. Hopefully others will post their experiences.
I think it should work, a calculator I looked at shows that the wheel would sit 9.5mm further out than the 55mm offset on my current wheels. At the moment I am running a 10mm spacer up front and a 15mm in the rear, so technically a 45mm up front and a 40mm in the rear. Thinking about that, the rears should fit with zero issues, but the 5.4mm arch gap loss may be problematic up front and may even rub ever so slightly in the rear. Not many people running 20's, maybe I should ask in the Polestar specific forum since they came with a 20" wheel. Using this calculator:
I think it will be close either way just from my experience with winter wheels on my Polestar. Sounds crazy but tire choice will play a role as well - tires with the exact same specs from different brands will have slightly different dimensions. The Tirerack site is very helpful for figuring this out.
Yeah Tirerack is definitely helpful with that sort of thing, I'm getting the feeling though that the 42mm offset simply won't work for the front wheels and that 5.4mm loss up front coupled with the 45.5mm offset may cause problems there as well.