The question put forward as the pretext for mine's closure does not answer my question at all—that question contains “who(m)ever” in a clause acting as an object, which I have no trouble with. My question deals with a possessive contained inside a noun phrase, which is much more complex. Therefore, I request that my question remain open.
Parents should use who(m)ever's last name is shorter on the form.
Should whoever or whomever be used in the above sentence? The way I see it, “who(m)ever's last name is shorter” is a noun phrase (NP) acting as the object of the verb use, while “who(m)ever's” is seemingly acting as the head of this NP (more specifically, as a possessive determiner formed from a relative pronoun—how the relative clause looks like, I'm failing to figure out—and the clitic 's, with another clause after that to modify the head?).
So, how should it conform to the sentence?
Should “whoever's” be used here? The following sentence makes perfect sense to me:
Whoever's (whosever) last name is shorter should be used on the form.
since “whoever” is acting as the head of the NP which is the subject of the sentence. However, here, “whomever” is acting as the head of a NP which is the object of the sentence, which throws me off and makes an argument for “whomever” appear more compelling in my mind:
Whoever's last name is shorter [...], parents should use it.
since the NP is replacing the object it, and the pronoun should be an object pronoun.
Is my grammatical analysis correct? Is there a general rule of thumb to use when encountering sentences like these? I am not a grammarist of any kind, so I'd prefer it if you kept your answers in simple terms! I greatly appreciate any and all of your thoughts on this matter!