1. True in most cases. With those size differences, you're talking more than a quarter of an inch. That's too much. However, a couple millimeters here and there is no big deal, as long as you're careful with the tube while installing. After popping the tire back onto the rim on both sides, use your hands to push the tire back from the rim and look to make sure the tube isn't pinched. Do this on both sides and it'll make sure the tire is seated properly, and that the tube is in the tire properly. You'll also be more confident pumping a tire up to 120psi if you know the tubes not gonna go BANG on you.
2. Try stretching the tire out before you put it on the rim.
3. Use the method outlined in step 1, and don't hop curbs, and you won't have pinches.
4. Yes. It'll rip the tube at the stem.