Keep in mind, what these tenses are. Unfortunately, these tenses are rather confusingly named, for learners:
"present perfect" : describes an event in the past.
Formed with the present of "to have" + past participle of verb.
ex: "I have seen the movie".
"past perfect" : describes an event further in the past.
Formed with the past of "to have" + past participle of verb.
ex: "I had (already) seen the movie, when my sister called me".
So, in your example:
"...what he has taught..." (present perfect) -- implies that he has been teaching something, in the past; that is, sometime before now. Maybe he still is, maybe he isn't.
"...what he had taught..." (past perfect) -- implies that he taught something in the more distant past, and probably stopped teaching it some time ago.
"...what he taught..." (simple past) -- also works. very similar meaning to using the present perfect; sounds to a native ear like the teaching has probably stopped.
All three possibilities are correct, but they convey a slightly different picture of what's going on.
In your second question: your reasoning about using "would" is correct.
In your third question, "was" or "is" both would work fine. "Was" shows that it was wrong (in the past); "is" would show that it is considered wrong (now). Both convey the wrongness of the teaching.