[ewashingtons]

I think it is pretty clear if it is heads up then either person who is in the hand can say whatever they want. In other words, table talk can be used as a strategy to try and manipulate the other person in the hand.
I think it is also just as clear that if you are not in the hand, you should not be talking about the hand. You should also not be saying anything that might influence one of the players who is actually still involved in the hand.
I think where it gets fuzzy, however, is when there are multiple people in the hand. In that case, everyone involved in the hand should not say anything that would influence one player if not all players were able to get that information at their same relative time in the hand. In other words, if there are three players in the hand (A, B, and C) and player A acts, player C should not say something to influence the upcoming action of player B.
In your examples:

"pot is multi-way on the flop and it's checked to the button who starts reaching for chips; the BB says "don't bet, you'll regret it" and indeed the button ends up checking; ethical, unethical, or unclear?"

Unethical, because the BB gave the button information that the other players in the hand were not able to act on for their first action in that round.

"pot is three-handed on the turn and it's checked to the button who bets; before the first player can act the button says "careful, you're gonna get check-raise if you call"; ethical, unethical, or unclear?"

Ethical, because no one else has acted on the previous action in that round (the button's bet) and the second player will also be able to act on that information.

"four-handed on the river, and the first player leads out; the next player says "you're bluffing" and calls; ethical, unethical, or unclear?"

Ethical.

The fact of the matter is in poker you are always trying to influence the actions of the other players in the hand with you. Attempting this verbally should not be considered that much different, as long as you are not revealing information about your hand to other players in the hand or giving certain players in the hand an informational advantage at any time.