Inverse warding

Ah, okay. This makes sense to me now. Thank you.

Oh, I see. The ward acts as if a small, impenetrable wall between the two.
But if that's the case, trackless step should not work: the magus would still push the ... dirt, mud, earth, whatever away with the pressure of his feet -- indirectly applied through the spell (a bit like ... you still leave footprints when wearing shoes, even if the shoe prevents the foot from ever coming in contact with the ground).

Also, it seems to me that in your interpretation you would be able to cause pain to a warded person by smashing a wooden chair on his head -- as his head, or parts of it, would be violently displaced not by the chair itself, but by the invisible force maintaining the distance between chair and head. Does that make sense?

I have no problem with wards preventing such damage. It's magic, not physics.

So if a giant hits a grog warded against wood with an uprooted tree:

  1. the grog gets "almost" hit (the ward keeps a small distance between tree and grog)
  2. the grog is sent flying by the violent hit (as force can be trasmitted through the ward)
  3. while in flight, the grog is completely uninjured, because magic prevented the damage.
  4. when the grog lands hard, he suffers an injury.

Not really. I'd say that the grog would be hit by the tree and get shoved back by it, but would be otherwise fine. Otherwise, the magic isn't doing what it's supposed to do. Again, it's magic, not physics. There's no need for conservation of energy or notions like a = F/m. The tree hits the ward, displaces the grog, and continues its arc.

That is one possibility. Another is that the tree hits the ward and is stopped cold there without having any effect whatsoever on the grog.
I don't think they had quite figured out conservation of momentum back in the 13th century. and thus there is no need for the grog to get shoved out of the way.