What actually causes the head gasket/bolt hole failure in the 4.6L Northstar?
 

98 DeVille. Do the threads simply pull out at random, causing overheating as exhaust gases leak into the coolant? Or does the engine actually have to overheat FIRST for some other reason, therefore causing undue stress on the block/heads, making the threads strip out? Just wondering if it's actually dependably preventable.

I would say it is preventable with proper maintenace. That being change coolant every two years. The real issue is overheating which can be caused by head gasket failures due to corrosion from old coolant. When engine overheats badly, heads warp with heat and pull the bolts. It is possible to have leaking head gaskets and not damage the threads and heads if caught. In a cast iron block head gasket failures are almost never fatal, but aluminum blocks are prone to warping heads and pulled bolts. Northstars seem to get a lot of publicity for this but any aluminum block is subject to these same problems made by any manufacturer.