Not arguing it, seems like you are covering everything. It is just so major, to have it unique to yours though. I was hoping for some one-off modem or something to try to help distinguish why yours does not work but so many other (apparently similar) scenarios do work (or are not posting if they are not working).
I always hated one-offs or unique-seeming problemd in my QA days... There was always "something" but tracking it down could sometimes take longer than the release cycle, then it would be "fixed" in the next one and nobody was any wiser as to WHY or HOW.
Not sure I can recommend or help much more. Hopefully Securifi can spot something in your logs, although who knows if the useful information survives downgrade, since you cannot send a log after upgrade (although maybe they could have you pull something useful even then to send to them?).