Its exactly as you call it: Exploit ... willing one even, since you cant simply do this "by misstake".

You cant effectively defend against all of them, and honestly ... if someone wish to ruin his game experience by this, i would say its his poor choice and i see no reason to spend both time and resources for trying to figure out how to prevent them from doing so, while keep other possibilities intact (like casting guidance, or other buffs). :-/
Shouldn't we have a timer to answer ?
Certainly not ...
This whole game is made based on "
the world will wait for your decision player" idea ... it should remain intact, either in all cases, or none.
Shouldn't in this exemple the other goblins attack if my characters enter the village before the dialogs end ?
Certainly yes ...
And if you try the same approach in Windmile, one of Worgs will spot you, alert the others, all goblins will attack you and conversation is ended.
This sounds like good example of solving the situation imho.
Shouldn't it eventually be possible only if the other characters are unseen ? (hidden in exemple).
Certainly yes ...
But that i would say in included in previous quote.

The first second you are seen > attack!!!