I recalled that whilst I had been browsing the Logitech support site I had seen some downloadable software called 'Options' for the M185 mouse, that I did not recall seeing on the corresponding web pages for the M215 or M515.This is where 'n' will probably have an upper limit, but it is at least 3, as I had already established that 3 mice could work simultaneously through a single unifying dongle. Thus, you would only need one dongle for 'n' devices - (say) 2 devices (a mouse and a keyboard) - to communicate simultaneously through a single USB port. The purpose of the unifying dongle seems to be simplifying and efficient, making optimum use of a limited number of USB ports: instead of having to have a separate dongle for several Logitech devices (e.g., a mouse and a keyboard) that you might want to connect to a PC, the unifying dongle enabled a 1-> many communications channel for a single USB port. Of the 3 mice I had, the M515 was apparently the only one whose specific 'original' dongle was also a unifying dongle.I felt sure that would not have been a desirable design objective. Unpairing a mouse did not enable that mouse to talk to its specific 'original' dongle, and it effectively left one with an orphaned and otherwise useless mouse. I felt sure that there would be software available to modify these settings, but the unifying software only offered a feature to either pair or unpair a device/mouse to a unifying dongle.So the pairing process is definitely to a sole, specific dongle (the unifying dongle, in this case). You can test this after the pairing (and I had already established this), by putting the specific 'original' dongle into another computer, and the mouse for that dongle will still be unable to talk to that dongle, whereas, if you put the unifying dongle into that same computer, the mouse/mice that was/were paired to it will still be able to talk to it.That would presumably be due to a modification in the mouse firmware. It seems that once a Logitech mouse has been paired with a unifying dongle, then that mouse will only talk to that dongle, and will not then talk to its specific 'original' dongle.This note for posterity and other seekers. I haven't been able to find any info on this so far, which is why I posted this curiosity and #f0dder: Many thanks for your helpful comments. What I would like to know is, if the mouse firmware has indeed been changed (as would seem to be the case, by deduction), then how can one revert/restore that firmware to its default initial state? Leaving the batteries out of the mouse for a while doesn't seem to do it. So, it seems that I may have just disabled 3 mice (or 2 at least), in testing out the Logitech pairing functionality. So, the unification seems to aggregate mice or other devices when it pairs, without discrimination, and it seems that the firmware within each mouse is changed on pairing, so THAT mouse can ONLY work through the unifying dongle, and henceforth THAT mouse cannot work through its original dongle as a 'Logitech HID-compliant cordless mouse'. I then re-pared the meece (using the Unifying software) to the M515 dongle. However, when I then tried to use M185 with its own dongle, it simply would not work. That seemed to work fine too, according to the UI report. The UI can display a report of the battery status (if the software can fetch the battery status) and the firmware version of each mouse paired. Then I installed it and, using the unifying software, paired the 3 meece to the M515 dongle.

They used the USB device 'Logitech HID-compliant cordless mouse'.ДЄs M515 was a Unifying dongle, I downloaded the Unifying software from Logitech support. They all worked fine, separately, using their 3 separate, individual/unique dongles. Using Win10 the other day, I experimentally took 3 meeces (Logitech M185, M215, M515). I wondered whether any DCF denizens could help to unravel a problem with this unifying process.

Logitech unifying software download