Sorry this reply is six months late, but I hope it still helps!
From what I've seen and read, "para quién" seems to be used when some material object is to be given to someone. E.g. ¿Para quién has comprado esas flores? Or ¿Para quién es el café con leche?
Whereas if this is not the case, and "for" really means "for the sake of" or "on behalf of", then "por quién" is used. E.g. ¿Por quién harías una locura? Or "¿Por quién arriesgarías tu vida?
Hope this is not misleading.
We don't seem to have a definitive answer from KWIZIQ TEAM for the distinctikon between "¿Para quién?" y "¿por quién?" The latter also translates as "by whom"
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