Ok there are lots of people on this forum who know more than me about muay thai, but until they find this thread I'll try to answer some of your questions.
I'd say it was a freak accident. I've seen or heard about that happening three times all together, so I guess it's very unusual. The guy defending the leg kick tried to block with his shin, which is the standard thing to do, and the kick obviously had a lot of power, which kind of is the whole idea.
I guess it's relatively slow if you're talking about kicking many times in row, but of course you'll get faster with practice. The video with the girl does not
display a muay thai roundhouse kick. In muay thai, you don't chamber your leg before kicking, and you try to hit your opponent with the shin, not the foot. I don't know if the karate roundhouse is faster, but probably... At least it has a lot less power than the muay thai roundhouse.