Having some axle confusion here – Trying to swap out a broken (cracked) freehub on a 2 yo pro2, quick release, with an older one, around 8yo. Although old, it’s come off a 142×12 axle that I put in approx 2 years ago.

The axle on the broken freehub is wider, and the old freehub won’t fit on it. Why the difference?

Sounds to me like the new hub is a Pro 2 Evo which has a larger diameter axle. You need the Evo freehub. Mine did the same and Hope replaced it FOC.