Can isfile() and isdir() functions both evaluate to False?

Is it possible for a file f that os.path.isfile(f) and os.path.isdir(f) both evaluate to false?

yes .. if you ask for existence os.path.exists(f).

EDIT: to answer your more detalied question ..

Would the union of B and C be equal to A?

Normaly yes .. if you can assert that none of them is deleted in the meantime.

Take into account that dead links are not exists, which means that os.path.exists(deadlink) of a (existing) link pointing to a dead end results in False while os.path.islink(deadlink) results in True, no matter if it points to a existing object or a dead end.