Wow… That’s a long post.
I know your intention is to make sure we have as much background information as possible, but I, personally, find that it is important to summarize with only 1 or 2 specific questions.
So let me highlight 1 question you asked and answer it, and then you can ask specific follow-up questions. Sound fair.
The answer is no. The SmartThings WiFi Mesh system is not capable of routing ZigBee nor Z-Wave over IP protocol. i.e., not over Ethernet nor WiFi.
Heck, I’ll answer one more of your questions:
Of course they can’t. There is no such thing as a “SmartThings sub-hub”. Only a “SmartThings WiFi sub-hub” and such hardware is not acting as a SmartThings Hub, so SmartThings Support cannot see them. Only the Plume platform can. Furthermore, Plume doesn’t know anything about SmartThings Hub(s).