Photos Keyword and People or Faces labels - duplication, conflict and error

I have two Macs, an iMac and a MacBook Pro (details below). The iMac is running on an external SSD because its internal hard drive was becoming unreliable. In most situations this set up is working fine but I have a problem with Photos on the iMac.

After the hard disk issues I had difficulty associating Photos with the iCloud sharing system, which resulted in a great deal of duplication of libraries and photos. This was almost certainly driver error - my fault - and I have resolved that, so sharing and synchronisation now works properly.

What doesn't work properly is the labelling of people's faces. The odd behaviour is:

1. In going through the library labelling faces in the People view, Photos has created Keywords for most of them, which I have not experienced before. (I note on another thread that someone had the opposite problem, that user-allocated keywords were being removed by Photos in favour of Face names.)
2. When entering a label under a face, the forward auto-text often offers a name that has already been allocated and an entry that I believe is coming from my Contacts app. This has resulted in some inconsistent labelling and unreliable searching. For example, a person who is in my Contacts and in lots of photographs has a listing in People and a Keyword of her first name. I do not recall creating this Keyword deliberately. If I search just on the Keyword (by adding it to a Quickgroup and pressing the 'Showing' button) I get 422 photos. If I start to search in the toolbar search box I get 696 pictures with her first name and 2 with her full name.
3. For one particular individual (that I have established so far) there is an even odder problem. He keeps being identified by Photos as his late wife. This goes beyond an error in the facial recognition. He has a Keyword of his full name (which I don't recall creating deliberately). Searching on this Keyword produces 5 photos: in one his face is present but not identified with a name caption, in two his face is labelled "unnamed" and in two his face is labelled with his wife's name. If I try to correct the labelling by typing his name all goes well until I press 'Enter' when the wife's name reappears. There does not seem to be an entry within People for him, although there used to be, and he has vanished from the People album. He is in my Contacts but his late wife is not.

I have concluded that there is something wrong with the indexing within Photos. To resolve this I have re-created the Library from iCloud, I have repaired the Library and I have restarted Photos and the Mac numerous times. After each attempt and correction I have allowed several days for the scanning and indexing processes to run, watching in Activity Monitor photoanalysisd running at 180% of CPU time on occasion. There is also a little glitch in the 'Showing' button, which sometimes has lines within the drop-down menu overlapping each other.

System: iMac 21.5 inch late 2013, MacOS Catalina 10.15.4, 8 GB RAM, 1 TB SSD running the OS boot disk and the Photos app and database.

Photos: version 5.0 (141.19.150). The Library contains 13,119 photos and 165 videos. I have not experienced the slowness and unresponsiveness reported elsewhere in respect of Catalina and Photos.

MacBook Pro 15 inch 2017 with identical software. This does not seem to be source of the problem but the unreliable indexes have now synchronised across, so the behaviour is the same.

Everything is reported as up to date.

Perhaps I should add that I have been using PowerPhotos to merge Libraries and resolve duplicates, but the issues pre-date my use of this app, which seems to work well and has been very helpful. My Pictures folder contains several Libraries, including migrated Aperture and iPhoto libraries, an obsolete Photos Library from when I started doing all this in March, a backup of the current System Library created by PowerPhotos and a duplicate created by me as a fallback.

I'm not sure where to go from here so any help would be much appreciated, please.

Stephen