Since Leina and Melora are optional components in the weidu install, can these two npcs be installed without installing nej parts 1 and two?

Not that I want to do it right now but, I may wish to do it on a future install when playing mods incompatible with nej2.

Well, they are optional for NeJ2 but not for BG2. It means they are optional components for the mod you are playing. To install them separately requires some additional work.