So, if I understand all this, if I 'import' it will duplicate the LR5 catalog data to LR6, but the LR5 catalog can still be read by 5? Where as if 'open' the 5 catalog with 6, it can no longer be opened by LR5?
No. In either case, the LR 5 catalog is unchanged and usable in LR 5.
Import adds one catalog's contents to another catalog. File->Open Catalog simply opens the selected catalog into Lightroom.