This is a question done because of the VERY frequent confusion about Planning and operational phase of flights. Some parts of regulations refer to one and some others to the second. In the Planning phase you need to prove that you have chosen a valid alternate acording to the regulations and all that is needed to prove this MUST be enclosed with the flight dispatch. If you can't have acces to this then you are not sure if there is an NOTAM saying AIRFIELD CLOSED. The dispatcher should provide you of at least on option to divert (unless you use an airfield with 2 independent runways...etc EU OPS) The CFP calculations just demostrats the fuel and time capabilities but that is only part or the story.
On the other hand you have the operational phase and if you have a problem or something changes or your passengers booked the wrong airfield (like it happened to me a few weeks ago) then the captain with all availabel meens must re dispatch the flight to the airfield that he considers apropriate and complies with all legal and aircraft limitations. Some people think that if you divert you must divert to the planned airfield and that is simply
and many more think that if you reach mimimum diversion fuel you must divert another
. Just take the safest and most logical decision is a good advise.