Guillaume,
I would treat this the same as Your Case B from your original post. Because the action is NOT head to head, and it appears to be a gross misunderstanding. I might even consider allowing Player 6 all options. I could also support a decision that would force Player 6 to forfeit his undercall by folding, or completing the bet to 55,000.
I believe Mike said, this would offer some protection for a player in the event of a gross misunderstanding.
In short, I believe that forcing Player 6 to complete his bet to 55,000 is too harsh.