So If I understand correctly:
- Before you started adjusting the gib you had .010" play in the head along the Z axis. Was this strictly parallel to the axis of the column? Then this is backlash, it might account for the problem, where is it from, the ball screw or the bearings that fix the ball screw, worth tracking down.
-Also after your adjustments you can fit a 0.0015" feeler guage in the ways, between the column and head in about 1.5 " deep.? This is no good. You can do other tests and adjustments but this has to be fixed. You should determine if the gap is due to a crack/bend or bad badly made parts. If it is bad machining you can re-machine the ways in the head. To get a proper fit it would be scraped. When the gib is properly adjusted there should be no gaps between the ways on the column and the mating way surface on the head, and similarly for the gib to column contact surface. This is acchieved by proper machining, fitting and scraping, not just tighter gibs.