I’ve now successfully (uninstalled, ‘purged’ and then) re-installed RW 8.7; and moved my add-ons one by one into the ~/Library/Group Containers folder as ‘Managed by RapidWeaver’.
The fact that I no longer get such messages as these does appear to suggest that there is some way in which RW retains a list of (non-existent) addons… now looking forward to getting back to work 
Thanks everyone!