I think someones is like peoples or persons.
different groups of someone, therefore someones. >_>
example: there was a person at the party. -regular
there were several persons at the party. -somewhat poetic but it may be used for persons of importance, to show that each is an individual.
same with someones I assume.
as for someone's, yes it is compound and yes it is the correct form here.
I think I'm loving the general writing section more than undergrad and term paper ones. ^^ hehe "ones"