MP, best hand South
Some may argue that the entire fiasco, if fiasco it is, stems from human South's systemic deviation by opening 3C, 2 HCP heavy (influenced by the best hand South feature). I disagree that this should have a significant impact.
The requirement for "2+ D" in the final double seems rather bizarre, but not uncommon with GIB penalty doubles, even at lower levels.
I don't know what 5D by East would have meant instead of 5C, but presumably 5C invites West to bid 6D if he has 19 total points (given that West's 5D sign-out limits his hand to the same definition as the 4D overcall bar the 19th total point). I would be interested to know how West's hand for 4D differs from hands that would make a direct intervention over 3C.
But possibly the strangest action of all is the 4D bid. Given that he only has 5 card suit when the bid promises 6, why would he not instead prefer to protect with Double, what with also having 4-4 in the majors.
Whether we would find the profitable 5CX sacrifice over 4M at these colours is doubtful.