I remembered an article from an older copy of rbc ministries' Our Daily Bread that may be somewhat relevant. After digging through my train-wreck of a room for approximately 5 minutes, I decided it was easier to google the article. I found the article here. Here's a quote of the first paragraph:
Many people who come to Marc Salem’s stage shows think he can read minds. But he makes no such claim, saying he is not a psychic or magician, but a close observer of people. He told writer Jennifer Mulson, “We live in a world that’s mostly invisible to us because we’re not paying attention to things . . . . I’m very sensitive to what people give off” (The Gazette, Colorado Springs).
I then go on to do a little googling on this "Marc Salem". These paragraphs from his Wikipedia page is of interest:
According to Salem, most of the people do not absorb the majority of
information that is available to them in their day-to-day lives. This
is especially true when people are involved in a game such as poker.
No matter how good an opponent is, there will always be some leakage
of information given off by the body. If there is an attempt to hold
it in somewhere, it will escape somewhere else. The numerous
interactions that go on around people often don’t even enter their
field of understanding. Salem states that if we train ourselves to let
them in and then correctly interpret the information that is
available, we can gain a tremendous advantage in a situation.
There are three basic mind-tools needed in learning how to read people
and detect deception. Observing means to closely watch for the visual
ingredients of deception and learn how to recognize facial expression,
body language and gestures. The second step is listening effectively
for what normally goes unheard. It means a system for picking up vocal
tones, sudden changes, unconscious sounds, word choices and silences
should be developed.1 Lastly, the information has to be interpreted,
meaning comparing current behavior to what has been seen and heard
before. Finding inconsistency in an action is the challenge and the
key. An inconsistency is often more significant than the action
Summing it up, Salem is not a mind reader or a magician, but he is
able to pay close attention to people's body language.
I decided to re-watch some of Merrit McKinney's tricks. First, let's start with the first trick he does, where he guesses that the man is having an affair with his wife's sister. Note that, he don't directly say the name of the mistress; rather, first he chants letters alphabetically, and, if you watch closely, the man's eye subtly twitches when he says the letter "J". He proceeds to mention some common girl names. When the man's eye twitches again at "Janet", he knew he got his answer. But the whole Florida, him having an affair in the first place, was credited to "intuition". A little bit of a handwave, really. Here's a quote from Merrit McKinney during the interrogation of how he does his tricks:
Rhodes: What exactly is mentalism?
McKinney: Tricks, mostly. Some science.Targeted guessing might be a most apt description. Along with some intuition and the occasional...Voices in my head.
Not very solid, yes, but it reflects what Marc Salem does.
Hypnotism is a completely different part, and this answer is long enough as it is, so I think I'll keep it short and simple. The point is, I'm not sure how that works, but the Wikipedia page as well as this link may be worth reading.
My personal, unprofessional conclusion: Merrit McKinney can plausibly exist in reality, although if he is real, he would be considered as an extremely talented and observant entertainer.
EDIT:I re-watched more of the film and noticed one scene where McKinney tapped someone's forehead, saying something like "Whenever you think about Janet, you'll see me naked, and that's not a pretty sight." It seemed to have worked. I have to admit that this part is probably a liberty by Hollywood.