Well, no prizes for guessing that it was the printing fix that scuppered my attempts at editing 
I found this post about exporting PDF files, which even has similar screenshots to mine, helpful:
And from this post it seems clear that real problem here is libre office-qt5.
I haven’t tried the suggestion to use
SAL_USE_VCLPLUGIN=kf5
but I have removed the line of code from the desktop file, mentioned in my first post. Editing still isn’t perfect (some of the tool buttons disappear) but at least it works. I just won’t be able to print anything quickly 
Here are some quotes from the Lubuntu home page:
“Your daily operating system”
“…a complete Operating System that ships the essential apps and services for daily use: office applications…”
" Lubuntu now ships the most advanced desktop interface, using Qt technologies…"
“Everything works out of the box.”
I’m not sure I can agree with all this when you can’t even print a written document without delving into the terminal.
Disappointed. 