Popay,
you seem to be torturing this to death in order to support an answer that you are simply refusing to accept despite you having asked the question ?
This has nothing whatsoever to do with aerodrome minima planning or fuel planning, and of course you as the commander are responsible for adhearance to FDP requirements. You use the word "limitations" to support the contention that there is no flexibility in the requirements, but as you know this is simply not the case.
As the commander you are within your right to refuse to excercise the use of your "commanders discretion" if you have reasons for doing that, however it is not prohibited for your company ( or its agents) to request that you apply your discretion, if on the day of the flight it may be required due to previously unforeseen circumstances such as headwinds etc.
You then argue that these flights can rarely be operated as planned and you have had it pointed out repeatedly that, then becomes an issue for the regulatory authority and should be brought to their attention if it hasn't already been by virtue of the numerous discretion reports that would have presumably been generated.
If what you say were the case then around the world gate agents would be telling the passengers "we cannot operate the flight today because the headwinds are actually stronger than the company thought they would be when it planned this flight". The reason that doesn't usually happen is because the flight time regulations have a number of "flexibilities" built into them. They have been covered already in this discussion but Commanders use of discretion is clearly one of them. It can be used prior to departure and to the limits specified subject to the criteria required. If you don't want to use it then that is a matter for you.
I am getting very lost by your arguement. As the commander you are responsible for adherence to all the planning rules. As commander you have the option to apply flexibility if you wish, where it is permitted (as in the use of commanders discretion to extend an FDP). You appear to want the regulations to state that "You are not permitted to dispatch on the day of the flight if due to actual circumstances on that day the flight does not fall within Scheduling FDP limits even though the previous planning schedule indicited it was possible". Unfortunetaly for you the regulations do not say this, the only get out clause you have is your refusal to apply discretion in a particular case. That is your choice you are the commander. The rules will not provide the concrete arguement you otherwise wish to fall back on.
Finally, despite your excellent use of English, it is obvious that this is not your first language. Even if it were there are many incongruities that can make literal interpretation difficult. Regulations such as this should avoid such shortcomings, however I think you are getting too hung up on the use of particular words in order to transpose them from one arguement to another, "planning" being one example.