J M, your opening questions identify ambiguity in interpreting CAT.POL. The options appear as to either use the dispatch factor in-flight – no further discussion, or to use some other aspect for the pre landing assessment.
In the latter case IR-OPS has the catchall – the pre landing assessment of landing performance should be judged as 'safe'.
Thus much of the debate above is about what is safe. This should relate to the requirements and guidance in IR-OPS, various advisory information, and an awareness of best practices in industry including accident statistics (those aspects which the lawyers will look at).
EASA publishes regulations (rules) and may provide guidance. The National Authorities implement the regulations, provide additional guidance, and oversee operations; the operator bears the final responsibility for safety – Captain.
There is scant EASA guidance for the in-flight aspects, but the European Action Plan for the Prevention of Runway Excursions suggests that FOLD (providing a minimum of 15% addition) is an acceptable interpretation of the requirement for pre landing assessment.
The UK CAA provides some guidance under winter operations –
“The operator is required to demonstrate that it can ensure safe operations, and by applying the same additional safety margin to the in-flight performance assessment as to the dispatch assessment demonstrates an equivalent level of safety. An FAA Safety Alert for Operators published in 2006 (SAFO 06012) recommends an additional safety margin of at least 15% on actual landing distance, except in an emergency”.
European Action Plan for the Prevention of Runway Excursions.
UK CAA Landing During Winter Operations.