PQI: Is a data engine required to return atoms in the latest form, or only historical forms?

In the PQI, we ought to state what is required of a data engine that has a store of Atoms of various versions.

Is it only ever required to return Atoms in the form they were originally submitted?
Is it permitted that a Data Engine only returns Atoms in the latest approved form?
The PQI says that some queries return Atoms and also says that certain fields can be omitted: In this case, the return might not be a compliant Atom (because it could be missing mandatory fields). Do we allow this?