So to continue the above with some of my use of Lightroom CC (hope it's of interest to some) ... I deleted the photos from Lightroom CC and (as expected) the files stayed on the iMac / Lightroom Classic CC; but they did get deleted from the Lightroom Mobile Collection visible on the iMac. In addition, it should be noted the original files were deleted from the Lightroom CC "local copy" folder on the laptop.
The files (safely on the iMac / Lightroom Classic CC) could easily be added to a new folder to sync again.
So tried to add more images ... 9 this time ... including the images I had deleted from the Cloud earlier. Interestingly it recognised them as the same images (presumably by file name) and didn't create duplicates. Even now the initial sync has finished, upload times aren't particularly good (combination of WiFi running at 300Mbps to AirPort Extreme connected to 1000Mbps wired network and onto 40/10Mbps fibre from Sky) and the 9 images (RAW from D750) took over 5 minutes to upload. Syncing of changes happens in a few seconds though.