No, but unravelling the correct usage here is tricky. Normally the verb agrees with the closest subject, which would yield You or I am mistaken, which sounds wrong; and so some people will say You or I are, which is barbarous but does not sound that wrong since we are used to hearing the equally barbarous "aren't I" ( which is a mistake inspired by reluctance to use the more correct but decidedly non-standard "ain't I"). Careful speakers will say Either you are mistaken or I am.
No, "will be had" is not a correct grammar. The correct grammar would be "will have."
Either form is acceptable.
I would say it was. It could be phrased- With you I shall depart. Either would be correct.
No, it is correct grammar, not a correct grammar.
"On a train" is correct grammar.
no_____If the sentence is You do do that (meaning You are in the habit of doing that) the grammar is perfectly correct and the sentence 'does have correct grammar'.
"July has just been started" is not correct grammar, instead the correct grammar is "July has just started."
Grammar.
This will be an exciting month is correct grammar.
The grammar is correct as it is.
No. The correct grammar is "would give"
Yes, that is correct grammar.