OK....you asked for my two cents. Let me see if I can clear up some of the confusion. MC is absolutely mono-embryonic. The three seedlings that I have fruited are very different from the parent and very different from each other. The seeds are generally underdeveloped and many or even most lack vigor and will not even produce a tree worth growing. So I really think it is unlikely that anyone would purposely plant out seeds of this cultivar in a commercial setting. So I think that variability is most probably a root-stock and general growing culture issue as relates to different growth habits of the trees and fruit characteristic variations. Of course I cannot exclude the possibility of limb sport or the propagation in Thailand of a similar looking mango that they labelled MC even though it wasn't truly the same genetically, but only looked similar. I don't know the source of Excalibur's grafted trees so I cannot comment on that. Their trees, while initially mis-labelled and named at the nursery, do appear to be the real thing from the look of the trees and fruit that I have seen.
Regarding Leo's tree in California, I do not know if he has had more than one source that he has grafted from but I can verify that he was sent grafting material of the real thing many years ago by some crazy mango nut here in Florida.