The magnet rotors were chilled and laying in snow when welded, mags never even got slighly warm.
The magnets are held in place with loctite green, so they will not rotate.
As to your welding first, then placing mags. How do you intend to get the mags on the pins without chipping them, it will be difficult to not have them slam into the rotor plates. It is easy to slide them in from the side, but putting them straight down will be a challenge, Good luck
Dbcollen