No, they are not related. Kirstie isn't related to any of the Ambridge families. I am quite fond of her, and thought the 'Philip' storyline a good slow-burner.
Having said that, she probably isn't a good judge of character - she used to see the best in everyone, and didn't spot Tom's doubts, or Philip's duplicity. I think she may swear off men for a long while!
She is a fantastic friend though, and I do think TA portrays friendships very well - although I haven't forgiven the SWs for dropping Amy.
Roy is, I'm sorry to say, a bit of a drip. He seems to allow folk to walk over him, and rarely takes the initiative. He may have left his racist past behind, but IMHO remains easily led. I'm not sure that he & Kirstie would be good for each other, although there's a real 'Harry met Sally' vibe there!
As for accents, I think they are realistic. I live in a part of the country that used to have a very distinctive accent, and it is disappearing.