The claw caps are one time use, and fall off with natural shed claws. Although I’m pretty sure it’s mainly for the front paws on cats due to excessive scratching/destruction, so four times for the front paws.

Yes to some degree.  Our cat still runs and plays and can still get on top of the kitchen counters.

Purchase based on the weight of your cat. If your cat’s weight is closer to the next size up, buy the larger of the two, as some claws are thicker than others, especially the back claws. 

I only recently bought Soft Claws for both of my about 1 year old cats. They're around eight lbs and I bought the mediums. They are working great. I heard about Soft Claws, however, from a friend of mine that had just gotten a new kitten (maybe about four-five weeks old, so pretty small yet) She had bought the small ones to put on her cat and had absolutely raved over them. So I think your cat would indeed fit in small Soft Claws. The good news is that if they are a bit big for her, she'll just grow into them anyhow!