Personally I think that PfMW is overpowered compared to mantle/improved mantle/absolute immunity. My preference would be to make PfMW only grant immunity to up to +2 magic weapons.
However, I realise that this would cause some serious problems for SCS (and indeed enemy mages in general) so I suggest an alternative:
PfMW grants immunity to all magical weapons and can be breached or dispelled (same as right now)
Mantle grants immunity to all magical & non-magical weapons and can be breached or dispelled
Improved Mantle grants immunity to all magical & non-magical weapons and can be dispelled but not breached.
Absolute Immunity grants immunity to all magical & non-magical weapons but cannot be breached or dispelled.
All versions would have a duration of 4 rounds. And there's a clear progression in usefulness as the spell level increases. By the time it gets to Absolute Immunity, there's basically no way of getting rid of it (it is a 9th level spell after all!)
I also have a comment on Imprisonment. I'd like to see a save added to this, but probably with a hefty penalty. Say -6 to -10.