New coins are minted after every 2 or 3 moves, depending on difficulty. You can keep tabs on them at the bottom of the screen (Red bar - 1 move, Yellow bar - 2 moves, Green bar - 3 or more moves) and strategize accordingly. If a column of coins reaches the top of the screen, you'll get a warning, the coins will flash, and you will need to remove enough coins to get back to a safe height. If you can't make a move, your game will end!
You could try advertising more, for example, by putting up signs around the neighborhood, posting about it on social media, or having people you know spread the word. You could also try doing it in a different neighborhood that might have friendlier people, or do it in a time and place where there's likely to be a lot of people walking around the area (e.g., near a church before the end of a mass).
What’s the catch? None, really. Cash back apps act as affiliates for many online merchants, which means that whenever you make a purchase through one of the apps, they get a small commission — but then, they give you a portion of that commission as “cash back”. For example, if I buy a pair of Nike shoes through the Ebates app (or website) and spend $75, Ebates may get a $10 commission but then they’ll pass $7 back to me. It’s basically a way to get sale prices on stuff that isn’t on sale!