I am actually asking about the spelling of this word, not whether it exists... which in fact makes it rather difficult to write about. I will spell it as "whoever's" for the purpose of explaining the question.
Take these examples inspired by colloquial US speech:
*Whoever's umbrella this is, please come take it!
Write down *whomever's name was on the form we got.
How, morphologically (and also plain logically) speaking, would the asterisked word best be spelled?
I've never seen it in written form outside of casual text-chatting, where it's oft rendered as I wrote it above, but this feels slightly off to me taking into account that who's is incorrect. Would "whoevers/whomevers" be more reasonable, if at all?
...I suppose it would also be relevant here to inquire why, exactly, the 's clitic/affix appears to be reserved for nouns in English and never pronouns (as would be in "who's", "he's", "she's", "they's"...?).