I would like to mention the flambeau school of Vilano here. They have made a whole school of indirect attacks and I think the thinking follows.
Look at Neptunes wrath and Waves of Smashing and Drowning. These spells can capsize boats and send the wizards into the drink.
Now the spells do not need to penetrate to be effective. I don't see how being thrown out of a capsizing boat is any different to being elevated on a levitating shield.
The magic works on the boat/wave/shield not on the magus. I suppose it could follow that in these circumstances the spell could throw or trip the magi with ease. I'd personally allow the Parma to resist the force of being squished by another object as the parma is essentially 'pushing back' against the force trying to squash them.
The way I see it (although it's slightly out of paradigm) is that the parma acts by acting as an opposing force against the magic, but the magical energy can be transfered when the target of the spell becomes the medium of the magic force.
Again we come back to Neptunes wrath where the spell throws the waves around, but what the waves happens to hit and move are also moved. There seems to be a degree of abstraction from the source of the magic, but as long as you are being effected by a 3rd for further rmeoved causal result then the magic need not penetrate.
1st Causal - Magic (working on unresisting water)
2nd Causal - Wave - (working on unresiting ship)
3rd Causal - capsizing Ship working on Magi.
so a metal shield could work as follows
1st causal - ReTe (working on unresisiting shield)
2nd Causal - Metal Shield working on wooden parts of shield
3rd Causal - Wooden parts of sheild working on lifting magi
or the magic carpet
1st Causal enchantment (working on part of carpet)
2nd causal - part of carpet working on Rest of carpet
3rd Causal - Rest of carpet working on magi.
I would also justify however that in cases where 2 opposing forces were applied to a magi directly or indirectly, where one is magical, then the parma acts as a buffer between these two forces negating the conflict.
for example lift the dangling puppett would lift a target against the wall with enough force to hold them there, but not enough force to crush them. However a magi can walk on a magical bridge however magically driven natural water/fire will dampen/burn a target but not hit them with force.
my point is that the target becomes the indirectly driven media so just like mighty torrent of water, the target touches the wall, but the force is resisted.
In short, the media needs to be at least 2 generations removed from the magic to move the target (which is easy with a little thought) but when this force comes into contact with an opposing force (such as a wall), the opposing force starts to directly effect the magi so reistance would come into play.