The prop saver usually uses thicker, stronger o-rings instead of rubber bands. If your motor is mounted in pusher configuration, it won't matter as much, though. If the prop pulls on the motor it might be an issue.
Yes, you got it right. Eventually you will want to get a prop adaptor for the motor, like this 3mm one as there are always reports of the rubber bands popping off in flight. Then again, I have flown my floater jet on a 4 cell with a prop saver, and have never lost the prop, so its your call.
I have a bunch of prop savers. Just about have them all in a drawer. I kept loosing them in flight OR they weren't the right size for the prop adapter or they WERE the right size and even though a balanced prop would get them vibrating and a whole bunch of other problems that I eliminated....
By using a regular prop adapter.
If I ram the thing into the ground I'm more worried about the motor shaft then a 2 dollar prop. If the shaft is gone I can order another prop with the new motor. Prop savers made since back when you had 10 sub c cells in the belly of your light and fragile airplane that you had to javelin launch and land at just under maximum speed because of the heavy batteries. If you want to try it yourself, take your plane as is, then add a pound of lead under the CG. NOW fly it.
You have to stretch the rubber band as much as possible.
I always stretch until the rubber band wont stretch any more when i put it on my propeller. The smallest rubber bands will only go back and fort 2 times but a little longer band might go some turns. I am never using the thicker rings as they need to much force to be tight with my setups.
Another more important issue is that the hole in the propeller must have a perfect fit to the adapter.
1/10 of a mil is to much gaps.