I think that the OP is asking whether, since there is only a single base in the rear rank of unit B, that single base can form a Line of Battle with both units A and C at the same time, even though the single base in B cannot be within one inch of both of the other units at the same time. If all of that is correct, then my assessment is that B can choose to move the rear base and form a Line of Battle with either A or C, but is not able to do so with both at the same time. That is, I have always played it as per the rules, i.e. that if no part of a unit (in the OP's case B) is within 1 inch of another unit (C), then those two units do not form a Line of Battle.
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