"When you open 1NT your hand is very limited and well defined."
When you respond to Blackwood, the number of keycards you hold for your response is very limited and well defined.
Yes, if you realize you made an error with your response you may reconsider.
I am one of the "old folk" who doesn't mind it at all, in fact I very much prefer it. I just think of it as another way of permitting kibitzers.
"that I was playing at all is something that I think should be private to me"
Does this ...
"We believe that South would have (and should have) bid 7NT also over a fast 6NT."
That is where we disagree -- I accept and respect captaincy. I don't think about bidding on over 4N//6N any more than I think about bidding on over 1N-3N. It saves my (limited ...
"a fast 6N indicates N has no doubt about what the right contract is"
Since South would [b]never[/b] bid over a crisp 6N, how can it not be the case that a slow 6N [b]could[/b] suggest that bidding on [b]could[/b] be successful?
An in tempo 6N says we have plenty of HCP/tricks, but we're missing a keycard. A slow 6N says something else. Am I surprised that the responder guessed right, that it was a level hesitation rather than a strain hesitation? Uh, no.
IMHO, you are not allowed to ...
"As it turned out their solution was to announce “I have half the deck” and then their partner looked at their hand and said “hey, I have the other half of the deck”."
The tempo allowed for them having all the keycards...
"And in that hand, if S chose to pass, then he is correctly interpreting N's BIT?"
If the auction were like here and chose to pass, he's not interpreting the BIT at all, he is doing what is expected on a captaincy auction.
If the auction goes 1N-hesitation ...