Well.. a good solution for those two problems in my opinion would be:
1) For those that use the /abandonmission, could have a cooldown, like, you could leave the mission but then you wouldn't be able to use again in 30 minutes or 1 hour ,or an agressive way to avoid this behaivor would be to penalize the player
2) For the griefers problem, a simple solution would be to tempban the players. Currently if you kill your teammates 4 times, you get kicked from the server and then you're able to join again (sometimes used to leave the mission). Instead of this what LO could do is, if you kill 4 teammates you would get 30 minutes ban, then if in the same week you kill 4 more, you will get a 1 hour ban, and so on, at the end of the week those bans would reset, so if you in a week just got baaned for 30 minutes, if you don't teamkill more in that week, in the next you get a reset.
Edit for solution 2: if i players get's 5 tembans in a week they would get a permanent ban for griefing.