So to sum it up, one has to differentiate between:
- Actual weather conditions
- Operation CAT in force (determined by ATC), which would normally be determined to barely cover the actual weather conditions, as higher CATs could exclude operators with more basic equipment and would reduce traffic throughput
- Aircraft capabilities (CAT II/III F/P F/O)
- Ground equipment capabilities
- Crew capabilites
Now back to Q2:
So the reason why a (quite high) DH is required for an autoland in CAT 2 conditions (performed by a F/P system certified for CAT3a) is that the overall performance (including for example ground facilities) is determined by the "weakest" part of the system, which in this case might be the ground facilities operating according to CAT 2 standards?
Another question that is still not really clear to me is why a F/P autoland system requires a DH to be established. The system should alert the pilot in case of a failure, who could thereafter intiate a G/A even at low altitude.
The ICAO Manual of All-weather Operation states that for F/P systems "a DH is used with the intent to have adequate visual reference to support a possible manual roll-out during the period following touchdown". So the DH is required as most F/P system do not offer rollout capabilites - is that the whole story?