I haven’t been following this discussion but wanted to throw this out in case you guys weren’t aware.
TorBirdy 2.3 encrypts subject lines but 2.1 is the version that made it into stretch. I wonder if it’s worth it trying to install from the Add-ons menu. I know doing so has caused breakage with dependencies in the past. (I’m assuming current guide still recommends install from debian repos). Sending subjects in the clear is a very easy way to shoot self foot.