Who picks up an item in the controlling players turn, in a column with more than 1 tribe? Is it the controlling tribe, or is it randomly assigned?
Items picked up by a column are randomly assigned (those darn zimbies fight and scrabble for it). The entire column is the item holder (since it is considered a single creature),
If that tribe already holds an item, is it forced to drop it to pick up a new item?
A tribe holding item A that picks up item B must choose which item to keep and drop the other (so, the tribe can keep their preferred item). In a two tribe column, the other tribe get's the dropped item. In a three tribe column, randomly assign the dropped item to one of the two other tribes.
Hope that helps!