If there is a posibility that one may have to change destination (fuel) and am aware at the planningh stage, I tend to use the RIF function.
The only alternates specified on the flight plan are destination alternates. There is only room for 2 and at times there may be many more. The aircraft commander can choose to divert to any suitable aerodrome at any stage of the flight.
My use of the alternates on the flight plan are basically as per the ops requirements. I.E. VFR or IFR within the specified requirements, I may not specify any alternate while at the other end of the scale, with destination below minima, two alternates are required and thus two will be specified.
The departure alternate (if required) is usually not recorded but I like to put it in a remark to keep ATC in the picture.
An aerodrome can indeed be made unavailable as an alternate at the planning stage not just by the aerodrome operator but also by your own operations team. However, it is a brave aerodrome operator that refuses use of the airport to an aircraft seeking to divert there during normal operating hours. More usual for them to point out that if one lands, there will be delays, no stand available etc.
Regards,
DFC