Merriam-Webster, definition 2b.
b : to control or play upon by artful, unfair, or insidious means especially to one's own advantage
Also see examples.
"She knows how to manipulate her parents to get what she wants."
A daughter consciously acting in a certain manner in order to get her parents to agree to something that they wouldn't otherwise agree to. Even something as everyday as a toddler throwing tantrums in a public place to get candy would fit this definition. If you consider Triss's persuading Geralt to "remake himself" to be a conscious attempt to stop him from remembering Yen, then it would fit this definition.
"He felt that he had been manipulated by the people he trusted most."
The realisation that you had been persuaded into a course of action by others who didn't have your best interests at heart. By this definition, if you believe that Triss deliberately involved Geralt in politics to further her own political career, it would fit the definition.
"The editorial was a blatant attempt to manipulate public opinion."
A single act can be manipulative. It doesn't need to be carried out over a period of time. The accusation of manipulation doesn't require the editorial to have lied, just phrased the truth in such a way as to cause the readers to think things the way that the newspaper wanted them to think. Triss choosing to allow Geralt to think she was his "Sorceress Lover" would fall in this category, as would the Loc Muinne speech if you think that Triss was more concerned with convincing her audience than with being 100% accurate.
Cambridge Dictionary:
http://dictionary.cambridge.org/us/dictionary/english/manipulation
the action of influencing or controlling someone or something to your advantage, often without anyone knowing it.
Please note that this is "control OR play upon" and "influencing OR controlling".
There are a few important criteria that each definition lists. The first example about the child manipulating her parents to do something they wouldn't do. It specifically left out the actual acts in that example so we can't determine the level of controlling vs artful, insidious or unfair. It therefore only sated that "something" happened that would have fit the definition. The example then mentions throwing a tantrum. But the act of throwing a tantrum is not manipulative itself. Otherwise every child everywhere would get their way by throwing a tantrum. Would you say that a child throwing a tantrum and then beinging punished for it manipulated his parents? Of course not. There was no gain. So an act by itself is not manipulative which leads me to the next part.
And the first definition requires it to be to one's advantage. The first "or" in the definition separates control or the rest of the definition so it applies to the artful part. This means that Triss had to have received an advantage. A relationship can be considered an advantage but it is an advantage for both sides IF they had free will. That is why the definition list "control" as the first part.
A parent is going to give candy to a child. The decision has already been made. The child throws a tantrum and the parent gives the child candy. Since the act of giving candy was not reliant upon the tantrum therefore there was no manipulation. Manipulation must have a cause effect relationship.
The definition also lists three other words (artful, unfair and insidious). I don't want to make this too long but in law each of those conditionals need to have a definition.
The next example "He FELT manipulated" is easily dismissed. There is no finding of fact as feelings are not fact based. Everyday people feel they are wronged but in about half the cases taken to court there is no actual finding for the plaintiff. The example never lists the actual acts. The last part is pure speculation and it requires a finding of "best interest" for Geralt. There is also an assumption that Triss got involved not for love but for her "political career". Neither has been established.
The last example once again doesn't exactly explain the act. Instead it is a finding. "The editorial was a blatant attempt to manipulate". It never said what was in the editorial. See I'm not denying there aren't things that are manipulative, I'm saying that none of the examples show the actual act that might have been manipulative. The examples already found that there was a factual basis but never says what that basis was.
Now I'm 100% positive that you would agree that simple influence is not the same as being manipulative. Else every reviewer, every critic, every politician, our spouses, our children, or beliefs are all manipulative every time they influence us. So simple influence does not arise to the required criteria.
Controlling is the simple best definition because it meets all the criteria. Influencing is too generic and has other clauses that will need to be met. That leaves us with "play upon" which includes other conditional tests that must be met.
Manipulation really boils down to "making" (via control, influence or other methods) another person do something that they would not have done otherwise. That is critical in order to prove manipulation. You can't manipulate a person if they are going to do it anyway. So the real question here is this:
Would Geralt have had a relationship with Triss if he had no amnesia? If the answer is that he would have slept with her then there can never be any manipulation as his own actions would have ended int he same result.
Personally, I'd consider that most of the major characters in both TW2 and TW3 are manipulative at some time.
That is exactly my point. Using a broader based definition then others are manipulative. And if someone says "I hate character A because they are manipulative" then they should hate character B since their hatred is based on the act of being manipulative.
It is not like she was the only one to fall into the same trap.