2) I've seen springs flying away several times, however I doubt RF's have anything to do with that. I have no idea what else can cause that though.
4) That's a very interesting effect I didn't think of. In fact no MCE is used there, although event 255 (commonly called MCE Event but used for things other than MCE's too) comes in useful. Download this. The effect you described is used for gold coins inside of pipes there. You should be able to see how does it work in JCS. I won't explain it exactly as it's kind of complicated, but for MCE Event with Event parameter 128 placing Belt Right below it teleports objects left, Belt Left - right. Different Event parameter values result in different distances and directions (left or right).
5) PU's are affected by gravity so you can just place it over a half-masked tile and then trigger it to become a full-masked one. Alternatively, simply place it in a half masked tile.
6) Belts. Obviously, they only allow you to move it left and right, and it will still travel up at the same time.
10) Another very interesting effect. It looks like a Gem Ring MCE (Event parameter 192) and most probably is one. At least I achieved something rather similar by placing it. I suppose its looks might vary depending on some weird conditions. Also, it results in Access Violation in 1.23.