Learn English – “Who to follow” or “whom to follow”

I'm not really sure what you mean by "dative" in English, as there isn't really an accusative/dative distinction - in situations where other languages might use a dative, either the accusative is used ("I gave him the book") or a preposition ("I gave the book to him"). However, the following might be helpful in articulating why "who" can be used, and may even sound better, where some insist on "whom" - whereas in other situations "whom" is still preferable:

To make my explanation clearer (at the expense of much precision, for which please forgive me) I'll refer to two "styles" of English - one very formal (in which the who/whom/whom prescribed by the style guides is compulsory), and one much more colloquial (in which who/who/who rules the roost, and "whom" is seldom if ever used). Very loosely speaking these correspond to English as it was both spoken and written in the past, and how it is most often spoken today; since trends in the written form often follow those in the spoken we might see current written English as being in a transition between the two.

Given all that, the sentence:

The man whom I saw yesterday was tall.

entirely follows the rules of the formal style, and is thus acceptable.

The man who I saw yesterday was tall.

entirely follows the rules of the more colloquial style, and is thus acceptable

However, the sentence

*The man to who I gave the ball yesterday was tall.

grates. This seems to be because it follows neither the rules of the formal style (which would have "to whom"), nor the colloquial style (which would instead have "The man I gave the ball to yesterday was tall", or a variant thereof), and is thus unacceptable in either. Similarly,

*The man whom I gave the ball to yesterday was tall.

also falls between both stools.

Note that this is largely handwaving, rather than a rigorous argument, but it's interesting to note that studies have been performed in cultures exhibiting diglossia (i.e. using "high" and "low" variant forms of what by some definitions could be considered one language, in different contexts) where subjects were shown words or sentences combining features of the "low" and "high" variants. It was found that some of the features were only weakly associated with one variant or another, in the sense that (say) using one from the low variant in a sentence otherwise fully "high" would not render it unacceptable; however other features were "strong" in the sense that a sentence containing features strongly associated with "high" and others with "low" would definitely render the sentence unacceptable. It is possible that, on a much smaller scale, a similar phenomenon is going on here (though of course it would be very bold to assert that this is the case without much more rigorous research!)

The traditional pedantic version of this sentence would be "It was he whom I voted for" (although a pedant who believed in avoiding stranded prepositions would instead say "It was he for whom I voted").

The object of the preposition for in the relative clause is typically analyzed as consisting only of the relative pronoun whom/who. The relative pronoun and the personal pronoun he/him refer to the same person, but they do not occupy the same position in the grammar of the sentence. The personal pronoun he/him is not part of the relative clause at all.

The "Let he/him who..." question has "he/him" as the object of the main clause and "who" as the subject of the relative clause. It seems comparable to your sentence, where "he/him" serves as a predicative complement in the main clause and "who/whom" serves as the object of the preposition "for" in the relative clause. In both cases, the standard prescriptivist viewpoint is that the relative pronoun should be inflected according to its role in the relative clause, and the pronoun before the relative pronoun should be inflected according to its role in the main clause.

"I chose whoever came first" is a different situation because there is no other pronoun before the relative pronoun: it is a "fused" relative. A fused relative pronoun is "supposed" to inflect according to its role in the relative clause. See this blog post: "For Whomever the Bell Tolls" (by Jonathon Owen, Arrant Pedantry). The actual, as opposed to prescribed, behavior of "fused relatives" seems to be fairly complicated: see F.E.'s answer to “Put me in touch with whomever created it”? for more details.

The verb be is not actually analyzed as taking a direct object, but a "predicative complement". Completely separately* from the issue of relative clauses, there is variation between "he" and "him" as a predicative complement: "It was him" is usual, but "it was he" has traditionally often been prescribed as a "more grammatical" form (based on the idea that the predicative complement should have the same case as the corresponding subject, which in this case is the nominative pronoun it). Sentences with nominative predicate pronouns, like "It was he", still exist to some extent as "elevated" variants of sentences with accusative predicate pronouns.

(*"Completely separately" may be a slight simplification: Barrie England's answer to "It is I who am at fault?" indicates that the use of "nominative" predicative complements may in fact be more frequent in present-day English before relative clauses that have who as the subject. But from a prescriptive point of view, there isn't supposed to be any relationship between the use of "It was he" and the presence of or case of a following relative pronoun.)