1. Water changes of 50% weekly is what most do. Test for nitrates with a good liquid test kit and when it gets to 30-40ppm is when you should do one. WC
's are as important as filtration and an AC70 will do. You can always add extra when you can but you should be OK, IMO
2. 10 cories are not too many for a 60g unless you're overstocked with other fish. They should be quite happy with 5 in each group.
3. Weird. My entries stay in it after I exit. I don't know on that one.