I honestly don't think a blanket will hurt a horse... Don't we put tarps on our horses sometimes? If they get scared of the blanket, they will toss it off themselves. Blankets don't contain mental spikes and won't hurt a horse if it steps on one. I don't think these girls are committing a terrible sin by laying a blanket across their horses, as long as common sense is used. If the horse is acting crazy at the mere sight of the blanket, don't put it on. Very simple.
Fake falling off... Whatever. As long as the girls don't hurt themselves. Personally, I don't like when a horse goes crazy when I fall off and don't see a huge problem with getting them use to it.
I do "stupid" stuff with my horse sometimes. I'll run around bareback, let her carry my jacket for me when we walk... I'll vault off on her bareback from the ground, lay on her like a couch, sit backwards in the saddles... All kinds of crazy crap. If I get hurt, it's my fault. It adds a lot to the horse experience to be able to act the fool sometimes.
Just being the annoying person with the differing opinion here.