Hello and Merry Christmas
I've defeated the Pulverizer in the arena and met Garry, completed my third faction mision (Coretech - Warehouse, ticked off in notes) and I've completed the rail crossing quest (ticked off in notes), in the process saving Buzzer and learning about Cornell and the Acid Hunters.
However whichever room in Coretech Rupert Simmons is in I cannot access without aggroing the entire building.
For what it's worth I'm not sure if I completed Rail Crossing 'correctly'. I killed all the faceless inside Buzzer's shop which caused the rest to vacate, but I didn't unlock Buzzer from the cage until after the quest had been completed. When I came back he was by the counter downstairs but would only give the overhead dialogue about the location of the keys to his cell. After unlocking his cell he walked upstairs next to one of the auto turrets and has been functioning seemingly normally since.
Here's my save file
http://www.mediafire.com/download/711ymy1w3am70aj/qweq.7z