Well, do note that this sentence isn't ambiguous - it wouldn't make sense with "mi" as "what", neither the conjugation nor the position of the verb supports that "hypothesis".
Also, in the sentence "What we are is not boys, but men", "What" wouldn't translate to "mi", rather "Ami", moreover plural. I don't think this sounds common (or even legit) in Hungarian but that sentence would be something like "Amik mi vagyunk, azok nem fiúk, hanem férfiak."
Also, é and i are like on the fence vowels. You still have híd, kín, cél, szomszéd, derék (the noun) as practically back vowel words. If i is the last vowel but it follows a back vowel (let alone multiple back vowels, like in optimális), I think it's more commonly considered a back vowel word than not, with é, typically both versions are used, depending on the speaker.