Howdy,
First off I can say that the standard BP D-plug is indeed 30mm (20mm + 5mm threads x2). I don't have a mini with me, but I assume that it is smaller than the standard, and consequently I would assume the 15mm number thrown about is correct. As for your other comments.

1. Somebody just provide the ID of the mini and we can see.

2. This could be a concern, but only the producer truly knows how large of an issue this is.

3. I know that folks use a 2nd o-ring with BP fittings on a GTZ so I would expect something similar here.

All things considered, what kind of cost difference would we be talking about between the radiator ala carte and the rad with 4 fittings? Also about what would the 4 fittings cost by themselves? If the difference in price between ordering the fittings seperately vs the built in cost of having them included is fairly small then there is really no reason to include them since roughly half of us wouldn't be wanting to use them. If the cost difference is substantial (returns to scale and whatnot) then they should be included. Just my opinion. Hasta.