Wow. I'm so glad I didn't have too much problems getting these achievements (Outside of the time it fuckin' took).
I did the (Drop through the shop floor glitch) to rack up the brunt of the miles for the explorer achievement, while netting the millionaire achievement first.
After getting all the achievements and finishing the game, the last I had to deal with was the Massacre and it didn't unlock for me at first and I was about to rage but all I did was quit to the main menu and it finally popped up at the screen where you can select the story, the two dlc's and the extras.
Hope no one else had to go through the initial shock we had when it didn't pop up, I was so glad to be finished with that game mainly because I was so tired of playing Jimmy's Vendetta.