1. The aircraft is monitoring it's current capability based on different system status.
2. Depends on the type of approach, cat 1 being based on baro, cat 2 radio.
3. Both are in reference to baro in MSL, Russia used to reference above ground (QFE) but has since changed, at DH you must make a decision, if visual references are not there, you make a missed approach. MDA means minimum decent altitude, in the old days, you never were allowed to decend below this height (baro reff MSL), nowadays they allow you to decend below this height when making a Constant Rate of Decent approach during the missed approach.