Like I said, My problem with isn't that they are flawed (or perfect) it's that they are bland to me. I think Mia might even be improved if Gran continued to be overprotective/interfering device (allowing Mia to be perma-horny), For example I hate the Mia interaction where she demands the MC make dinner, Gran could fill that role, demanding the MC step up as provider to her daughter & grand children, cockblocking him if her conditions aren't met (When Mia does it, my first reaction is "fuck you, I built you a house with my bare hands and you are nagging me already!!!"), A cockblocking Gran could also encourage mischief using shape changing magic to get past her (prolonging the dating phase & improving the variety of sex scenes).
(not to mention sneaky sex is the best kind of sex... it gets boring too fast, when it is too easy)
I agree Gabriel could be a fantastic device to add some flavour to Beth's narrative, I would have like to have him be obsessively over protective to the point Beth has had no real experience of sex/men, that's why she finds herself drawn to Samson, she has no idea why his cock fascinates her so, it just calls to some unfulfilled desire deep inside her.
No need to be so literal, as a basic archetype "real" brothers aren't required, other stableboys/villageboys could have filled that role (until her father chased them away when he realized that their motivations for wrestling had changed). The main point of the Archetype is she is a tough no-nonsense "boy" in a womans body, being overwhelmed with weird sensations/desires that nobody ever taught her about and she has nobody to turn to for guidance.