Turns out we DID have a frozen pipe inside the house, and boy do I feel silly for not having found it!  Brandon had someone from the water company come test our pressure at the meter.  They said the meter was fine, so we must have a frozen pipe inside the house.  When I took the heat gun to the pipes last night, I did the whole section of pipe that runs near the front of the house close to the foundation, but that didn’t seem to fix it.  I was specifically avoiding about a 6 inch section behind a beam because it’s a very old wooden beam and it’s covered in coal dust so I didn’t want to risk setting it on fire with the heat gun.  Well, turns out that’s exactly where the pipe was frozen!  Brandon tried again with the heat gun this morning and sure enough, the pipes cleared.  I guess when those previous owners 60 years ago wrapped that pipe and stuffed all those clothes into the crack between the front porch crawl space and the water pipe, this was exactly what they had in mind. 

Consider me newly educated in hindsight.