Sorry, I don't see it. Where exactly is the connection? It would just make as much sense, claiming she's Bayman's estranged daughter or even Zack's...or how about she's an illegitimate child of Bass and Tina's half sister. That's the reason she was searching for him. Or she might not even know it, but feel a connection.
Okay, lemme back up, from the original joke reasoning, to my slightly more solid current reasoning.
Originally, I picked up Mila because she "inherited" the missing Leon's mount tackle and beatdown.
Then I noticed she "shared" a few other moves with him (back kick, overhead punch, leaping overhead, etc), so I joked that she must be his daughter.
Then I thought about things for a minute, in a "what if way". It never made much sense to me that Lauren, Leon's dead lover, was a local of the desert, being a vaguely european woman with a name like "Lauren". So I figured "huh, what if Mila WAS really the child of Lauren". Would/could explain why a) She shares a LOT of traits with Leon, from movesets to goals to personality, b) why she idolizes Bass as much as she does, as a "Substitute father figure" due to rejecting Leon, blaming him for Lauren's death, c) putting the Leon/Bass, Mila/Tina, Tina/Leon, Mila/Bass tag teams in a whole new light of father/daughter pairs, d) Mila has no other named character relations besides Tina. It's not that she has no relationships with anyone else, its that she's actively rejecting/ignoring it, e) The nature of Lauren's death was never explained.
OOH on that last note. New idea that seems to fit in as well! Maybe the reason Bayman considers Leon a deserter or whatever, is because Leon was being targeted by an unknown party, and targeted his family as well as himself. He chose to try to protect his family over his military duty. Lauren was still killed, so maybe part of the reason Lauren and Leon wanted Leon to be the "strongest", was to watch over Mila, to make sure she wasn't killed as well. But with her still blaming Leon for her mothers death due to not knowing all the details, their relationship is strained at best. Hell, maybe the reason Leon hasn't been part of the actual story since doa3, is because he's been off dealing with the third party, making sure they don't go after Mila, by going after them first!
It is a little shaky, but it's also entirely possible and plausible as I see it. So for now, i'm subscribing to the theory that Mila is Leon's daughter, who blames him for her mother's death. Things just fall into place too well, to me.
None of those other characters you listed happen to actually fit in the role at all, like Leon does.
