Thank you for your note concerning the fifth example of an error regarding case that I give on the first page of my Breviary of English Usage. I so much enjoy discussions of the language; so few people, including academics, seem to care anymore.
It is no doubt a risky business to criticize the KJV translators, but it seems to me they must have been mesmerized by the accusative in the Greek text they were translating, which is (correctly) governed in that context by the infinitive einai. In English, too, we use the accusative as the subject of an infinitive: e.g., “I thought her to be a fine teacher.” But if we used the past tense instead of the infinitive of the verb, we would have to change to the nominative case: e.g., “I thought she was a fine teacher.”
One could of course choose to translate Mark 8:27 more literally (and awkwardly) by preserving the infinitive in English, and we would have: “Whom do men say me to be”, with both pronouns—the personal and the interrogative—in the accusative case. But I do not see how “I” and “whom” could ever be yoked in one phrase, the beauty of the Authorized Version notwithstanding!
A handy test is to change an interrogative sentence to a declarative sentence to check for the proper case. By shifting the word order, the sentence in Mark 8:27 becomes “Men do say that I am ___”. What are we going to put in the blank? It becomes clear, does it not? that the second pronoun must be a predicate nominative: “I am who” and not “I am whom”.