That is broadly my experiences. You need a bit more cement to compensate
for the bits that have already hydrated.
In the cement mixer, you tend to end up with large lumps that you can
remove by hand, and a a lot of fine sized lumps, which rather replace
the sand as the matrix, slightly weakening the final strength. That is
really not an issue for most uses - if the bag can be mixed at all, a
usable mortar results. If in doubt use some fresh as well. Overall
setting time is not affected. either parts of the cement have set, or
they haven't. They are not 'half set' in any meaningful way.
so unless you are casting stressed concrete members I would not be
overly concerned. My experience is that either the bag is 90%+ usable (
makeye a crust round it which can be broken up ) or its 90%+ unsuable
(its a fused mass of cement where its been exposed to a lot of damp).
The recoverable cement fom the latter case is too small to be worth the