No, this is not true, you don't get penalized if you leave after joining a group in progress, go try it if you want. Since you don't know the circumstances in the new group SE doesn't force you to stay when you go in progress.
Already did, been penalized for it. Admittedly though, this was some time ago and it's possible they changed the rules since.

Well something has to be done about troll behaviour, and the reason I put the stuff in bold in is to try and stop the get out of jail free card by getting kicked or disconnecting until the end of the dungeon. It's harsh, but I'm not sure what else we can do to combat trolls. If someone legitimately lost connection and can prove it to the GMs, then it'd be reversed. I agree, I forgot to include that statement in my original post. But that's the thing, we need to find more improved ways to combat the trolls. If my ideas don't work (as you say they don't), what else can we do to combat it?
The thing is, there's really only so far you can go to "combat trolls" while still providing an enjoyable gaming experience. 72 hour bans for things that may not even be the player's fault cross way over that line.

This is why they don't exercise "just ban everybody and let Hydaelyn sort em out" as a solution for getting rid of RMT.