These are FAQ items discussed by the MFA subgroup for possible addition to the FAQ or a future reworking of that document.
It is, but this is a poor chocie in most cases because it ties internal practices to an externally-imposed definition that may create problems down the road. Within the enterprise, there can be constraints on MFA usage due to issues such as a need to fail open during outages, special treatment for VIPs, very long "Remember Me" bypass policies with solutions like Duo, and so on. Maintaining flexibility is usually paramount, while federated interoperability requires more strict adherence to externally defined rules.
Serving both ends is difficult or impossible with a single <AuthnContextClassRef>, so the advisable course is to create a local value in the organization's own URL namespace and use that value when interacting with internal or contracted services, reserving support for the REFEDS MFA profile value for federated use. While in many cases this may be trivial because they may be handled identically by the IdP, having the second value allows them to be treated differently if they have to be.