What word can I use for a person/researcher/field who likes to put words in people's mouth?

single-word-requests pejorative-language

Solution 1:

A might be a:
misinformer
misguider
misleader
misdirector
misstater

All of which say that A has not stated the facts are they are. These are not words often used, as, today, liar is too commonly used. None of these words state that A is being intentionally dishonest, but, rather, mistaken.
If the idea of dishonesty need be applied:

disingenuous Dictionary.com

might describe A. Disingenuous is not as strong as liar, but is strong enough to offend some people.

A is being disingenuous about W

is essentially what you describe.

Solution 2:

The Devil can cite Scripture

The best of sources can be distorted to insert a personal interpretation. Origin: Spoken by Antonio in Shakespeare's The Merchant of Venice:

Mark you this, Bassanio, The devil can cite Scripture for his purpose.

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