Apple photos merging 2 people's faces

I've been doing a tidy up of my photos library lately. I have a lot of family group photos. Sometimes I've been correcting wrongly identified faces. Sometimes I've used the 'add face' option in the info box and typing in a name of a person in my people album. Afterwards I've had to go and merge several faces of the same person.

Now I find that Apple photos has merged my mother and me into my People face. I see this was a problem in the past. I've gone through and isolated my mother's photos in an album called 'wrong faces'. Then I've done 'this is not me' for her photos wrongly in my faces album.

Is there quick way to rename all the 616 photos of my mother that are in the wrong face album? Or do I have to go through each photo and rename her.

Can someone suggest what is causing this and how I prevent this from happening again. It's happened a few times with two different people. It's time I don't have to go correcting this all every time. Or should I give up on the Apple faces function as it's just too glitchy.