Dooku was probably one of the strongest force sensitives alive during the clone wars (surpassed by his current and former masters, and maybe a few Jedi like Windu). This would mean he had strong connection to the force.
Read up on the force ability Force Sense (Force Sense)
Force sense was among the most basic of Force abilities. It could be
used to feel another being's feelings, the future, ripples in the
Force caused by momentous or traumatic events, impending danger and
the presence of the dark side.
See how Force sense is a very basic ability and is not only to sense emotions or the presence of another being, but also to know about any dangers to the life of a force sensitive. This ability is seen pretty commonly in the saga. Anakin has visions of Padme's death, Yoda is able to sense when two clones are about to attack him, Luke is able to sense that his friends are in danger, and so on.
If I am permitted to state a Legends example, then in the Return cinematic trailer of Star Wars: The Old Republic, the jedi master Kao Cen is able to sense a danger even before the Sith Empire's fleet arrives at Korriban.
If the above doesn't seem satisfactory, I do have another explanation. I would like to point out that it is not stated that the poison dart used is in fact less lethal (not that I know of) and there is a possibility it was extremely lethal to a normal being. Let us not forget Dooku is a Sith Lord at this point. We have seen that force users generally have a stronger will and more important than that, they are able to hold their own (Anakin in episode 3, or even Maul post episode 1, were literally cut down and yet managed to survive).
So even if Asajj had used the most lethal poison at her disposal, it is not guaranteed Dooku would just drop dead.
This is from the official plot summary of the episode:- (Nightsisters)
His senses dulled by Ventress’ poison dart and the cloak of
invisibility which surrounds his attackers, Dooku is caught at a
disadvantage but fights back furiously
Nearly beaten, Dooku, who fakes being completely drained of energy,
instead channels all of his rage, and, in one extremely powerful blast
of Force lightning, he evicts them from his palace and disrupts
As for the latter part of your question regarding Talzin's plans, read this:-
But Mother Talzin is content: With this attempt at his life, she
reckons that Dooku will now be more anxious to get himself a new
assassin and bodyguard to protect him. As a result, she contacts the
Count and suggests a replacement – a male Zabrak warrior from her
people – an offer Dooku decides to take.
It seems Talzin wouldn't have minded if Dooku would have died. However, she is not displeased after he manages to survive, as she senses an opportunity there as well. Do recall that later in the series, Savage and Ventress try to kill Dooku together, but fail.