They don't lift themselves off the ground with their arms when they "skitter" - at most they would elevate their frontmost body a little, but they would generally only do so if the surface they were on was somehow unpleasant to touch - but their claws are utilized when they move, as suggested. But yeah, you get the idea...

Aemoten would target its other anchored arm? Poor creature... at that point it would probably lose its grip on the guard, who would just fall back onto the floor, whereas the effort the yth put into trying to pull the guard down would propel it backwards and down the stairs. By then it would be extremely weak and incapable of catching itself, so it would let its body go limp as it fell, bleeding profusely all the way down but remaining silent (yths rarely make noise normally)... and that would really be all. It'd be too weak to move much after that, let alone try to escape or fight, so it's pretty much out of the picture.