It sounds like your chainline is too far outside with the 113mm spindle and the XD cranks. I think it would be worth it to put a cartridge bb on there with a shorter spindle, probably 109mm. This will probably allow the Mountech FD to work properly as well as give you the proper chainline-
Which is disappointing... I like to follow directions (like the ones with the XD600 crankset) and have the result be as anticipated.
But, restating part of my original post:
Also, it looks like I could fit a penny between the drive side crank and the end of the taper on the spindle, would a 110 or 107 spindle work? Where, exactly, do they get shorter?
If I have a couple of mm of clearance on the drive side now between the crank itself and the non-tapered part of the current UN73 spindle, will a shorter BB be shorter in the right places? If I stick with shimano, will a 110 or 107 be a bit shorter on the drive side? Will the protruding parts of the BB (not the spindle itself) be appropriately recessed?