After trying to get "Till Death Do Us Part" three times in a co-op game (which failed all 3 times) and then getting it instantly when trying it in a single player game I'm wondering if there are any other achievements that you can only get in single player games? (I'm suspecting "Pink'd" to be one of them)
I received "Til Death Do Us Part" in multiplayer, but neither of the two members in my party did. The reason has become clear in the patch 1.0.2 notes.
All players in a party should now receive "Till Death Do Us Part" even if they don’t have Haedrig Eamon as their active follower
I suspect that any other achievements that seem difficult in a group are not on purpose (barring the follower achievements obviously), and are bugged.
While I can't comment on the specific achievements that you've mentioned, some notable examples of achievements that can only be earned in single-player play are those that require you to have a Follower active, such as I'll Be Your Wingman.
Certain conversation options for achievements are only available if the character is following you.
I had a hard time getting a few of Leah's conversations (in act 1) because I was always playing in a group.