So what is a Buddhist and what makes a person a Buddhist?
This is a question that can have various answers depending on how one might define what a "Buddhist" is or what makes one a "Buddhist" in the first place. To avoid that altogether, let's forget about the word "Buddhist" for now as it did not exist at the time of the Buddha and let us look to the Tipitaka...
Then the Blessed One, having understood Brahma's invitation, out of
compassion for beings, surveyed the world with the eye of an Awakened
One. As he did so, he saw beings with little dust in their eyes and
those with much, those with keen faculties and those with dull, those
with good attributes and those with bad, those easy to teach and those
hard, some of them seeing disgrace and danger in the other world. Just
as in a pond of blue or red or white lotuses, some lotuses — born and
growing in the water — might flourish while immersed in the water,
without rising up from the water; some might stand at an even level
with the water; while some might rise up from the water and stand
without being smeared by the water — so too, surveying the world with
the eye of an Awakened One, the Blessed One saw beings with little
dust in their eyes and those with much, those with keen faculties and
those with dull, those with good attributes and those with bad, those
easy to teach and those hard, some of them seeing disgrace and danger
in the other world.
Having seen this, he answered Brahma Sahampati in verse:
Open are the doors to the Deathless to those with ears. Let them show
their conviction. Perceiving trouble, O Brahma, I did not tell people
the refined, sublime Dhamma.
Then Brahma Sahampati, thinking, "The Blessed One has given his
consent to teach the Dhamma," bowed down to the Blessed One and,
circling him on the right, disappeared right there.
We see here that the Buddha saw that people could understand the Dhamma, regardless of if it would be easier or harder for them to do so. In seeing this, he decided to teach it (obviously). Let's focus on the last verse. He is essentially saying that people can attain Nibbana as he, but only to "those with ears," meaning only those who correctly hear the path can attain Nibbana. If the path was incorrectly heard, one could stray from it and thus never reach the Further Shore.
Therefore, people can hear (and subsequently follow) the path incorrectly, but will never be able to attain the goal as a result and people can hear (and subsequently follow) the path correctly, but will attain the goal as a result. Fast forward to when the word "Buddhist" was developed and you just replace the word "people" in the previous sentence with "a Buddhist."
Thus, a Buddhist and what makes a person a Buddhist is if they are following the Dhamma. Now if the Dhamma they hear (and subsequently follow) is correct or incorrect, that is another story, but in short, one can come to know if it is correct by direct experience to verify it (per the Kalama Sutta).
In my opinion, one should not worry as to what a Buddhist is or what makes a person a Buddhist, it all depends on if they are following the Dhamma, not what they are identifying as. What one should worry about is if the Dhamma they hear is correct and will lead them to reach the Further Shore (which they can come to know through their experience).
So was Devadatta a Buddhist?
Devadatta heard (and subsequently followed) the Dhamma correctly as the Buddha himself expounded it to him in the first place (as far as I understand). The problem is that he took what he heard and distorted it. When he did that, he was not following the Dhamma, he was following his own views.
In that sense, he was following the Dhamma until he swayed from that path and onto his own distorted one, making him a Buddhist at one point and not one at another point.
If he was a Buddhist; and a Buddhist can believe & practice incorrectly; are all humans Buddhists?
Devadatta was a Buddhist until he stopped following the Dhamma for his own distorted views. One would not be a Buddhist if one is not practicing the Dhamma that the Buddha expounded. All humans are not Buddhists because all humans do not follow the Dhamma, only a percentage of them do. Now whether or not the Dhamma one follows is correct or incorrect, that is another story, but (again) in short, one can come to know if it is correct by direct experience to verify it.
If he wasn't a Buddhist; and a Buddhist can't belive wrong things & practice incorrectly; then why do you call ie Mahayana and Theravada followers Buddhists when seeing that they believe different things and pracrice differently?
Refer to my answer in the previous question.
I hope all of this helps. May you be well.