Wishes some times come ...
Fully understand your design/idea that all is managed by Calibre.
Have some vacation coming up soon, so will do the backup rutine, and try out your plugin.
I'm currious about the books not transferred by Calibre, but expect they'll always be placed outside Collections.
Hope the "merge" collections idea is possible, and will be implemented, and I would like to help, however my programing skill's are not up to this.
Any other help I could provide, you got it.
Thanks, and have a nice weekend All !