I’ve heard this story before…
Personally, I bought a SmartThings Link for NVIDIA Shield as my hub last November, and it never worked with any of my Xiaomi sensors. I decided to switch to a v2 hub in late December and they paired quite easily and have never dropped their connection.
Since Xiaomi sensors aren’t officially supported, SmartThings support won’t offer any help based on them not keeping a connection. I did find out that the SmartThings Link hub uses different firmware and very likely a different ZigBee radio, and support confirmed that the Link is on a different development schedule than the v2 hub. The Samsung Connect Home also runs on different firmware from what I understand. This is something to consider - it may just not work with Xiaomi devices at all. Maybe with a future firmware update? Who knows.
To answer your questions:
Yes on the first question of A). However, it’s better to refer to a ZigBee hub as a “coordinator”, because that’s what they’re called. A ZigBee router is actually what a lot of people refer to as a “repeater”. Each ZigBee network can only have one hub/coordinator, and multiple repeaters and end devices. But ZigBee coordinators use something called a PAN-ID, which makes each ZigBee network unique from others. Coordinators/hubs self-assign their PAN-ID, but will use a different PAN-ID if they find another local coordinator/hub using the ID that was chosen. So this means there can be more the one unique ZigBee network in the same location. In theory, you should not have problems if your neighbor(s) also have a hub with ZigBee.