Can you get the achievements for making a specific choice during the story while you're in someone another player's game? If you can, will any of you guys want to start a new game so that we can grind through those last few story achievements? I would really like to keep using my leveled up character. If not then I guess I'll have to make a new character.

I can personally confirm that you can in fact do that, I had done 'The Belgian Problem' and saved the building and then hopped over to help my friend - he blew up the building before realizing there was a choice and I snagged the achievement.

I have a related problem where I have no gang members to taunt and can't progress my challenges.

@Protoroll:

I need to get all the co-op cheevos. Are you on a console or PC?

@Ravenlight: I have the 360 version.

There's only 2 missions that give you trophies/achievements based on your decisions.

The Belgian Problem and the last mission in the game.

The last mission in the game is the only mission that can be replayed through the game's offering in which case, making the other decision wills till get you the alternate trophy

For "The Belgian Problem", yes you can. A friend and I tried to do this a couple of weeks ago and I did get the achievements for it while in his game. I'm also assuming this can be used to get both items in a late mission where you have a choice.