Is there a table of supported hardware, that contains info about the USB-connection (or ethernet) on these devices. Like, which have data-lines connected, can the device electrically do host and device mode? Can I use a POE2USBC adapter, that presents itself as a USB-network device to the camera?
Ability to filter on those columns would be great.
Is thingino using the Ingenic linux kernel 3.ancient SDK version, or do they have/use something newer?
The manual, as OP said, does not offer any explanation, why the device might show up with an additional MAC/IP at the upstream switch port, and which services it might offer. OP sounds knowledgeable enough to be able to exclude the possibility, that the additional MAC/IP could be from one of the PCs, like e.g. when playing with VMs using an internal bridge in the Hypervisor.
Maybe the device has a bigger "cousin" device, that includes "control via APP", and this feature was not properly/fully disabled on this one.
Was the network port bridged to both PCs all the time (as the description makes it sound, or did only the "active" PC get a functioning network connection?
Could you tell from the FDB of the upstream device, if there were more than two MAC addresses active on the port?
Did you (hopefully) open it up and make PCB pictures before chucking it?
Many a soundcard supports changing jack "direction". Here's a StackExchange answer from 2012, on how to do it with the GUI tool `hdajackretask` : https://askubuntu.com/a/911961
Why did you not just login to the device, and switched off "Broadcast to multicast", or changed the destination address?
Edit: Some brands of Network-KVM use this, so that you can control the target device from another device, like e.g. an App on a tablet. That way you don't have to stand next to the target device in the noisy and cold machine room
Well, where do you draw the line? Is e.g. the SmartTV an IOT-device, or not?
If you want the TV to reach the Internet for some things (e.g. Netflix, Youtube, WhatEv), but also isolate it from your other devices, block ads, yet be able to use the UPNP-mediaservers in your network, and want to use the phone to control it for the things HomeAssistant cannot yet do, then you'll run into some difficulties. They can be worked around, but will need intimate knowledge of protocols and such.
Also some of these issues can be worked around by e.g. using HomeAssistant also as UPNP-MediaController, which I haven't gotten around to set up yet.
But "true", some categories of smart devices can be nicely sequestered into isolated VLANs.
Which is only the first half of the story for many installations.
If something at your place uses SSDP/MDNS/UPNP/etc to communicate/find_each_other, you'll then have to start doing multicast bridging/rewriting and other hacks. And for those, there is no one-recipe-fits-all.
It solves the Usecase, where not all BT(LE) devices in the house are reachable from the main HA, and you need remote "pickups" for their signals, and then want to integrate them back into the "master instance". It also solves a bunch of other problems, like integrating some sensors of the HA of your holiday home into the HA of your main home.