Try not to worry too much. Most cats do just fine. Yes you need one placenta per kitten, but if you get there late, or you miss any of it, she might well have eaten the placenta. Again, don't worry, the placentas are good for her. I've had goodness knows how many litters in my time (well, the cats have!) and the total number of problems giving birth can be counted on the fingers of one hand. You'll know if there is something wrong.

Lots of kittens come out tail first and those take a frighteningly long time to come out, especially if they happen to be the first in the litter. I've never known one to get stuck. I have had three cats that had to have caesareans because progress stopped half way through and it was obvious there was a problem, but that is a small percentage of the number of litters I've had.

Yes you will certtainly need to change the bedding when they are born, it will be disgusting. Then change it every couple of days. If possible, do this while Mum is out of the next, it's much easier!