Thanks. I misunderstood your suggestions.
However, the Network Diagnostics procedure would now seem to be moot.
See post #18396 above pointing out the procedure which seems to have corrected most, if not all, of the problems I've been experiencing.
Network Diagnostics are, indeed, moot for the moment, but I still recommend that you check in SysPrefs > Network as I suggested.
Even if your connectivity issue has been solved, having "Modem" as your top item would not hurt in the least and may, in fact, help something that you haven't even realized needs help.
I've read your posts detailing Apple's approach to your issues, and I've certainly got my fingers crossed for you, but I dunno...
Some of Not all the issues you've detailed
just don't seem to have been vulnerable to the steps you've taken.
Good luck, though!