Fortunately, this hole didn't need anything near that precise... I managed to force a good M8x1.25 bolt through it most of the way (to establish an initial thread), then hit it with a tap using a small crescent wrench (not enough room for a proper tap without pulling all kinds of sh*t out). I lucked the hell out and managed to re-thread it, and applied "adequate" torque to that bolt for fear of stripping it out.
Oh well. All major problems are solved, and the pinhole radiator leak seems to have corrected itself (though a new radiator is next on the list before I move back to TX in a few weeks). My AC works far better than it has before, holds a charge, and I managed to resolve a number of squeaking/rattling issues thanks to careful step-by-step re-assembly.
Then again, only time will tell. Thanks for all you guys' input - if the heater core goes in the near future, I'll put on my kitten stomping boots and then take it to the dealership to deal with this farking BS (unless I have access to an air conditioned 2+ bay garage/shop by that point).
***EDIT***
For those of you stumbling across this thread while seeking information about your running lights/corner markings and interior lights not working (yet signals and headlights do work), please check out the following thread:
http://www.naxja.org/forum/showthread.php?t=1022965
If you've tried every switch from every local auto parts store, yet none seem to work (with the exception of a few that work between knob positions), chances are you've blown the small 20A fuse in the PDC (in the engine bay) that is responsible for the running lights, as well as the bus (you might notice a code is thrown as soon as you turn the ignition to the "on" position). While looking in the engine bay and standing in front of the vehicle, the fuse of interest is immediately to the left of the bottom-right-most small blade fuse.
Oh yeah, and disconnect your battery before plugging a new headlamp/headlight switch into the harness (regardless of the position of the switch). You're almost guaranteed to blow that fuse every time.