Nothing new to report
I am still struggling to get any progress. I've given up with the section of Apple support that I've been dealing with so far, and have now contacted another department to see if they can help me move forward.
One of my apps, Standards Check, has received an automatic update from Apple. The description says the update is "signed with Apple's latest signing certificate". This sounds like it's related to the stuff I've been trying to resolve, but Apple has not offered any information about what's going on. I don't know why it is only this app so far, why no others have received the same treatment.
That's all the information I have at the moment. Still trying to get a resolution, but currently unable to either update existing apps or release new ones.