Not clear what is "cannot add the existing library to a group"... I can move the existing libraries between groups. Is it what you are trying to accomplish?
As for consistency between mobile & desktop editions, we are in a period, while the desktop edition is still new, when both editions are being enhanced, and the desktop edition is being upgraded with features in the mobile edition. In fact, I'm not sure, but some features may be added first in the desktop edition and later caught up in the mobile edition.
A small development group is putting its shoes on after the race has begun. It is unfortunate, but that's what we have for a time. Presumably, the editions will ultimately coalesce and conform to each other.
However, the platforms ARE different, and mice don't work like fingers. There may be features or implementations that will always work tablet-like on the tablet and desktop-like on the desktop.
NaN definitety occurs due to that duplication happening in Android. Therefore JS working fine. Now, the task is to detect why the duplication. That's the Google Sheets sync meddling rather than Memento, in my opinion.
I'd unsync everything and verify consistency in names first. Re-write scripts **manually** and if necessary, try to re-create fields under different names, and sync only after everything works fine in Android.
As for library grouping problem - still not clear.