Best practices for shell libraries (specifically error-checking)

[reiner otto]

... * should a good library do error-checking for the caller, ...
Of course. In my old days (I wrote my first machine code more than half a century ago) it was a basic requirement, that every software module/function has to be self contained, not doing any assumption regarding external environment. Which also includes internal validity checks. 
It is a pity, to read your statistics regarding so many exploits to be discovered because of wrong or even missing validity checks.
Looks like, old principles are forgotten.

Cheers,
Augustus