Hmmm, tough to say really. Looks like she might be, and if she is going to have babies probably in 5 days or so. Make sure you have some floating plants for the fry to hide if you are planning to sell them or let nature take it's course. I'm not an expert though
Quite a long way. Livebearers tend to look as though they've swallowed a box just before they give birth - their abdomen gets almost square. A young females with few fry might not get as square as she will as she matures.
She might be but at an early stage. I wouldn't say that she's so young. But what's also common is that a number of females when moved to a new enviroment (for instance, when you just bought them) may also drop a smaller batch when pregnant even if she had larger batches of fry before. That's quite normal.