I got out of my meeting to a voice mail from the person at the credit union I have been working with; she found the offending portion of the form and has re-faxed it to TSP. So I didn't need to undertake a bus journey out to Northgate after all. Much relief.
Still, the letter from TSP said something about "original signature" so I called the TSP line. After getting thru amazingly rapidly, the person I spoke with said a faxed signature was ok, and agreed to annotate my account. She said it should take ten business days for things to process.
Ok, so that only leaves the insurance payment as the giant glaring problem, and my mother has kindly agreed to loan me enough to get them to not cancel my coverage. I am willing spend some time sitting on hold to let them know their check is in the mail, or better yet see if they will do an electronic payment; I am on hold even as I type.
So, there appears to be some hope that the light at the end of the tunnel is not in fact an oncoming train. Some.