Would a set of winter tires that fit a 2017 Prius V fit on a current Prius Prime? Secondly, would a toyota roof rack that fits a Prius V fit on a current Prius Prime 2020 model?
I think NO to both questions. 2017 Prius V has 215/50R17 or 205/60R16. Prius PRIME (any year model) has OEM 195/65R/15. IIRC, Prius V also had a different bolt pattern 5x114.3 than most other Prius 5x100. I am less certain about the roof rack, but if it is OEM roof rack for Prius V, my guess is not going to fit on PRIME.
The rack for the 2020 Prime is P/N PW301-47009, which replaces PW301-47005 and fits Primes from 2017-2020. On the Toyota parts pages they list the models those parts will fit, and the Prius V is not among them. While the fact that the Prime rack won't fit a Prius V doesn't definitively prove a Prius V rack won't fit a Prime, it suggests that it is highly unlikely.