Despite the width of the middle of the base, I can move into the adjacent diagonal midpoint square of a peanut-base duo that is positioned diagonally, correct?

I.E.

D
XD

DX
XD

As long as the D squares are considered adjacent to each other. If the X squares are blocking, elevated, ect then you wouldn't be able to do so.

I'm not sure what the question is. Are you asking if you can move through the double based character?

I think the question is whether or not the X square in the original diagram can be occupied. The peanut base doesn't exactly leave enough room for another base, but it mostly does.

Anyway, yes, it's perfectly legal to put a character into the square alongside a diagonal peanut base.

Judging from the context, I think this is the question also. Although, mebbe there should be some room to move through the squares of two (or one) figure duo bases...