Hahaha, you're a legend man. Yea I am, I've pretty much done Tier 3, only have to finish of the last remnant. Tier 2 needs quite a bit of work. And my tier 1 needs quite a bit of work also, so I decided to start with South park =)
1) the achievements are currently glitched... kinda. however, the glitch is easily avoidable. simply make sure you obtain the 200 gamerscore in the original game before downloading the DLC. after obtaining the 200, then you can download the DLC and obtain the final 100 for a total of 300.
So delete the dlc and load up a level and play it and finish it and it may unlock.