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 original is probably eliding a word.
I think this works:
You are she whom I love.
'are' is a linking verb here; 'whom' is introducing an appositive phrase and is the object of 'love'. What's missing in the original is the predicate pronoun ('she', 'he', etc.).
Best Answer
"It's me/I they want!"
You may be confused because of the existence of sentences like the following:
Here, the subject is "I." The complement is the relative phrase "whom they want." The relative pronoun "whom" is prescribed in this context because it is the object of the relative clause.
The sentence you're asking about has a different structure:
This sentence is definitely not equivalent in structure to "They want me," so just because "me" is the object of that sentence doesn't mean it's an object in your sentence. I actually don't remember if "it" or "me/I" is considered the subject in sentences like this (I'd guess "it" because the verb agrees with it), but it doesn't make a difference for your question because the main verb is "to be". Prescriptively, the complement of the verb "to be" is supposed to have the same case as the subject, which in this case would be the nominative.
Unlike "whom," "I" and "me" are personal pronouns, not relative pronouns, so it's not possible for "me/I they want" to constitute a clause.
Since "me/I is not part of the embedded clause, the case of the pronoun is just determined by its role in the matrix clause (similar to your example "It is I whom they want" or the situations discussed in the following questions: Which is grammatically correct: "Let he who..." or "Let him who...", Is it acceptable to start an emphatic sentence with "It is he who…"?).
So the prescribed form would be “It's I they want!”
But "It's me they want" sounds grammatical to me, and I'm pretty sure to many other English speakers, so that's also OK (see Which one is correct to say: "It's me" or "It's I"?).
Interestingly, though, "It's I" is still more common than "It's me" when followed by a relative clause starting with the subject pronoun "who," according to Barrie England's answer to the following question: It is I who am at fault? Possibly, the reason is because many people find sentences with first-person verb agreement but not "I" (such as "Remember me, who am your friend”) jarring.
“Who is it you asked?”
This is just the question corresponding to the statement "It is __ you asked." When you form a question, the prescriptive rule is that the interrogative pronoun takes the same case as the corresponding word in the equivalent statement. (In normal speech in real life, the situation is more complicated since we can say "It's me" but not "Whom is it?")
Since we established that the pronoun in "It is me/I you asked" is not a grammatical object, we would use "who" and not "whom."