A final update.
I reset the router again, and then connected the extenders and other devices one at a time, refreshing the devices list on the hub management page each time.
I found that, as additional devices were added, some of the IP addresses of devices already added remained fixed - but one (or possibly two) of the entries' IP addresses changed each time. The common factor was that these were the extra "shadow" entries in the table. Each time, the entry for the "unstable" address showed the IP address of the device lying highest in the pool.
If I attempted to change the IP assignment of these "shadow" addresses, I received the warning that I was attempting to save it on an address that was already taken.
I have come to the tentative conclusion that this is merely the hub manager showing an incorrect address for an entry that actually has to be the same as its duplicate. I've changed nothing, and will see how well the network performs over the coming days.
