I don't think I'd have hangups with a little person my age though, no matter how old she really looked. I'd just hope she'd have the sense to wear an adult hairstyle and other signals of obvious adulthood. What do female little people do so people don't see them as children? Males can grow beards, but females don't have that option.
There was a TV series "Little People, Big World" some years back. The females age normally, like anyone else. The only difference is that they're shorter and sometimes suffer chronic musculo-skeletal problems.
How can you have an adult consciousness in a kid's body anyway. Isn't your entire brain different?
Ever read the first 3 Dune books by Frank Herbert? The characters of Alia, Leto, and Ghanima Atreides all had adult consciousnesses in children's bodies. Mind you, they were born that way.
As for the "situation" O'Brien couldn't be blamed too much for being uncomfortable but it's likely after time they would have adjusted and found a balance that made it work. Any physical encounters likely would have waited until Keiko's body matured more and O'Brien got over any societal ickiness over it. Society itself likely wouldn't give their situation a second glance considering the kind of world they live in. She could be in with aliens and all kind of goofy shit being possible, O'Brien could explain to any second-glancers with a "transporter accident." Looker: "Ah! Gotcha!" and then they nod at one another and continue on their way.
Miles might have had problems early on, but as Keiko matured physically, he should eventually get over it. The real problem would be Keiko's relationship to her own daughter, Molly. That would likely take a longer time to sort out, while Miles would probably be unconsciously thinking of both of them as his daughters.
I never liked adult Keiko, so this could have been a golden opportunity for her to grow up again into a nicer person!
That scene where Riker and Picard hug is just priceless!