Be Wiser! DF commence withdraw penalty affects your party.

[Fyce]

1st case: Well, thank you for the info. I guess it's that way to prevent an uncomplete party to still fish for "in-progress" tags... Still, it's a bit harsh.

2nd case: You said it, it was your fault, and that's right. Nothing to add.

3rd case: I'm pretty sure that you can immediatly report the party leader to the GMs. They probably don't have the power to remove a strike from your account for the day, but at least, if enough trolls are getting reported and get a tell from a GM, the practice will stop.
For the exceptional case of a new player trying to do a specific dungeon, I'm pretty sure the GMs can take action accordingly.