We seem to have it sorted out now, so with
@TaishoBee's permission I'm going to summarise what I did in case it's useful to someone else in the future.
I don't own the hair that TaishoBee was trying to delete, and I wasn't aware that it has several parts which conform separately, and can be mixed and matched as required. In other words, my instructions were understandably incomplete, and although TaishoBee did the edits perfectly, she didn't do enough of them, and some parts of the hair remained. That was easy enough to fix.
When I was looking through the files, I noticed something suspicious. The beginning of a PZ3 looks like this - there's a section that defines the UNIVERSE actor and the ground, followed by lights and cameras.
However I found another section buried within the figure definitions which sorta kinda replicates that block:
That shouldn't be there, so I took a chance and deleted that too; and the result was successful.
That spurious section was in the original file which was giving trouble, even before any editing had been done. I had made a comment to TaishoBee that 16GB of memory on her laptop should be more than enough for a scene this size. I suspect that this corruption had happened at some earlier stage, and it was this that was responsible for the problems and not the hair as such. We'll probably never know. Anyway, we'll see how the scene turns out, fingers crossed!
I recommended that TaishoBee retain a backup copy of the scene file at this stage, so she can backtrack if anything goes wrong later on. If there's one piece of advice that anybody can take away from this thread, it's this: keep snapshots of various stages in your project. Give them version numbers so you can go back to an earlier, happier time if something goes wrong; or even if you simply change your mind.