"Most of you would do the right thing if you/they knew"

A perusal of the numerous examples afforded by the ngram "most of you would" shows that when the pronouns and possessives apply to the group identified by "you" in "most of you", it is never the case of the third person being chosen; it is always the pronoun "you" and the possessive determiner "your". (never "they", "them", "their").

This should perhaps not be taken as the last word on the question. That seems to be the most logical choice when the speaker thinks proper to impute the state of knowing to the whole group out of which a subgroup corresponding to "most of you" is defined. However, an anaphoric refernce to only the selected group, or in other words to "most of you" would appear to be properly made through the use of the third person pronouns and determiners.

In British English you can never switch between pronouns in the same sentence, so whether or not the example here with they is ambiguous, it wouldn’t pass muster.

In American English it appears common to switch between one and you — the most usual alternatives — and this view is supported by this online article and this one.

I do not know whether American English allows switching between you and they, although in the example quoted, as already mentioned, it is ambiguous and therefore to be avoided.

This is the correct answer.

Most of you would do the right thing if you knew

You can't switch between you and they. The former refers to the people being addressed. The latter to other people not being addressed. The two terms have logically distinct referents.

That's the end.