1. It's difficult to say, especially when you don't know what they have done on the two occasions that you 'polished' your car. They might have used a very aggressve compound (that cuts a lot) or just very mild polish. There is this electronic tool that we call a 'paint thickness gauge' that can measure the thickness of the paint. From the readings, we can 'guesstimate' how much clear coat is left and whether or not it's safe to do any more compounding or polishing. You can try finding a place that have one but, as far as I know, not many detailing centre have/using this. If you can't find any, I have one in my possesion. Maybe we can meet up or you can join us in one of the detailing TTs that we regularly have.

2. Stone chips are a PITA. Nothing much can be done. Even if it can be filled and repaired, up close, it can still be seen. But it's better than repaint. Unless there's a terrible amount of them and looks horrible, then repaint is the only way. There's only one method (that I know of) that can offer good protection from stone chips - Paint Protection Film (PPF). It's something kinda like a thick plastic wrap. And, as usual, the better ones are expensive. You're looking at the price of around RM300 or more for my Waja front bumper alone.
There's a whole book written about the questions that you asked. They're really the basics for detailing. But to put it as simple as I can: