Who the father may be (beyond speculation), we have no irrefutable proof, - but there are some things that are seemingly logical.
I don't think most would deny that if Casey could have been getting child support all this time, she'd have been all over it. After all, she had NO qualms in overtly stealing from friends, parents, or grandparents. Therefore, why would she have any hesitancy abstracting money LEGALLY from someone? So, either she had no idea who the father was or he was someone who was unable to pay her due to some sort of backlash (ie, Child Services).
Likewise, it goes completely against her nature to have a deceased father of her child but not collect SSA. I think trying to convey ethics, morality, or compassion, to any of KC's motives is pretty moot, by now, to even the most naive.
The hinky-pattern of the immediate family's ability to boldly lie to "save face" and to circle the wagons in defense of those lies, is also a moot point as it is forever engraved in an epithet's granite.
I think that KC very probably knows who the father is. I do NOT think that he is dead. I also do not think that it is a simple case of the real father just "doing the math" and coming forward (as I would bet the house that KC would have long done so herself). There is, of course, the implication that the father is a relative and, while any ugly thought, cannot be glibly ruled out as it is a viable possibility.
What's reasonably left? There's rape without legal follow-up by partner(s) unknown, rape without legal follow-up by partner(s) known, consensual liasons with partner(s) unknown, consensual liasons with partner(s) known, - including incestual relationship.
While rape could possibly explain some things, KC would assuredly have told her closest of friends. There is not a peep regarding her ever having broached the subject with anyone. Consensual "ships in the night" partner(s), drunkenly promiscuous or not, again seemingly "fits" her lifestyle and history, - but, again, this doesn't follow her OTHER history of getting money by any means possible. The only thing she's shared with friends is a possible LA involvement. Do I trust her to necessarily disclose the truth? No more than I would negate everything she says. But, there ARE some things that DO "stick", and KC can ALWAYS be counted on to be true to her nature.
Anonymous sex orgies? None attested to or implied by school chums or older partiers (at least not during this time), - surely, we'd have heard something by now.
Though, there are a world of possibilities, there are not a world of probabilities. That something is being covered-up regarding Caylee's father, beyond the wagon-circling story foistered by KC and CA (about as palatable (and obvious) as simply being "fat" at Uncle Rick's wedding), is just another story. After all , "lying isn't a crime", per the CA lexicon.
There is a reason for the smoke and mirrors, and only so many probabilities. The most prominent with the most "sticking-power" is the most distasteful and ugly. It is not uncommon. What, if it is the truth, that WOULD be abnormal, is the depth of attitudes necessary to cover it up to "save" face and all those involved, the self-justification, the denial. These "attributes" run rampant in the Anthony household (nearly automatic) and would not be any stretch for most of us who've kept abreast of the social and personal interactions of CA, etal.