The bolded word here is the BESTSELLER. This kind of complement is a direct object complement. The adjective overnight denotes to the object bestseller which is an adjective. Another adjective complements another adjective, thus, the term direct object.
"spurn" means to, "reject with disdain or contempt."
gllllll bestieeee
I think its "Whose" because 'Who's" is shortened for "Who is"