Hub mesh is a protocol for one hubitat hub to use devices paired to a second hubitat. Pertinently, it has nothing to do with z-wave or zigbee mesh networks.
They have to be removed. The only way to do it with a C-5 (or lower) is to use a secondary z-wave controller, like a UZB stick, and PC Controller from SiLabs.
@danabw has documented this process in detail. Here is a link to his instructions:
Likely. It matches my experience from several years ago.
Yes. And that is what you are observing. The situation is alleviated by reboot, which clears the z-wave radio queue. And then over the course of the day (and depending on device usage), the queue gets filled again with transmits and re-transmits
The C-7 provides greater visibility of mesh details along with some approaches to remove ghost nodes.