I add this only for the edification of those interested in the final result. A phone call to Apple help (who spoke real English) eventually solved the problem, though I had them stumped for a while too.
After importing all my bookmarks from my iMac, I attempted to place the required new bookmark in a folder called Entertainment, and it steadfastly refused to enter it, or so it seemed. What turned out to be the problem was that for some reason, still unknown, instead of placing the new bookmark in the Entertainmnet folder I imported, a new folder called Entertainment had been created. When I found that, sure enough there were eight copies of the URL I was trying to import. So in the end it was operator error, though I still can't figure out why it just didn't go into the imported Entertainment folder.
All's well that ends well. I'm still unable to point to any faults in the two Macs I've bought in nine months. I'm a convert.