Teammates and takebacks
The new policy re: takebacks isn't really new policy, it's just newly codified in policy; it's what many senior judges (e.g., GP and Pro Tour Head Judges) already did. The change isn't how we want to handle takebacks, it's putting it in writing that we can lean on when players question our sanity. :)
And, it doesn't apply to teams. Each player is responsible for their game, and thus their “takebacks”.
In your example, Isaac, the crux of the question is really “what do the opponents do?”
If they don't want A to change based on what B said, we can point to policy and say “you gained information before you could change your action, so we can't allow a takeback.”
If the opponents don't care, we won't get involved.
d:^D