[Swiss1000]

For me personally - as i described in the beginning - this security topic is not a problem.
I am accessing my dad's drive as well as my own, we're too close to each other to ever consider this a security hole.
I get the point that once you start sharing your library with someone else there is going to be a security risk - but finding a proper solution for THAT sounds like it is going to be 1000x more complex than what i described (just remembering the google drive account per library, as Mike said, optionally enabled), if feasible at all. And as mdavej already said, once someone got the copy of library you cannot revoke access to it afterwards anymore anyhow.

Sharing a folder in Drive isn't complicated! It's a basic feature used by millions of people and a method used by ALL network administrators. 
If Mobile Sheet were to break this rule to accommodate a request, it would be truly absurd.

I think you don't understand the scope of your solution and that it's detrimental. Remembering one path per library solves your problem.