Further to my last post, I've worked out that when attempting to use the Volume mounts the volume that gets content shown is always the first volume that Android finds and mounts. It might be the internal drive or an external USB drive, whatever Android decides. Any further drives that get found and mounted do not show content by volume name. Content per UUID is recognised. One of my USB drives is a Seagate Hub backup which can act as a hub for two further drives. I attached two portable drives on this and they were recognised by the Q10 as additional volumes. Moreover, with the patch applied both UUIDs were listed in addition to those already attached making a total of SIX identified drives including the internal HDD. I could view the content of every one from within Kodi. The total number of volume names recognised was five, I.e. four of the five attached plus the internal HDD but again only one volume could be read from these mount points.
So, it seems that to accommodate more than four external drives, even if using the volume names work for you, mapping your library to the UUID mounts is the way to go. I got six total UUIDs seen and viewed but I have no idea what the limit is, if any.