The reason that Luc could take over Sasarai's Rune is probably because he was still in shock by what Luc told him. Geddoe, on the other hand, was simply overpowered. Chris and Hugo just didn't have enough experience with their runes to begin with, so they lost it easilly.Sarah can use magic that are undocumented... so her magic is really another thing. It might not come from true runes...
If I remember correctly, Geddoe's Rune was the only one that both Luc and Sarah were needed for.
Its confirmed that she could use the True Runes without bearing them. And her summoning magic WAS from a Pale Gate Rune. She just didn't have to bear it to use it.
Her illusion powers, however, seem to come from her Sindarin knowledge and/or staff(wich is called Illusion, just so you know). BUt I'm not sure about that.
Oh and this is going off-topic at a very fast speed.