Boiling down the point of a work of narrative art to a general statement can make it seem obvious to the point of being frivolous (e.g., "power corrupts" or "family is important") and we have to remember that the psychology of the work and the profundity of our experience is what's making the REAL point. Bearing that in mind, I think the ultimate point of 'The Owl Service' is something like "Love Over Hate"-- more specifically, if you allow yourself to be consumed by hate you're f*cked. That point is made to the reader more forcefully and painfully by experiencing our hero unexpectedly defeated by his hate, allowing the other guy to save the day because he is able to let go of his anger/hate for the sake of his love. Easy for him, we might say, he's middle class and has a bright future etc. And that's part of the point: Yes, Gwyn has it harder than Roger; Yes, the social forces acting on Gwyn's future are dire, his options more limited, his fate unfair. We sympathize with his anger, we appreciate it. And it's STILL his undoing, it's STILL bad. That's why he's a tragic character, why the book is so haunting.
As to your other questions: My take is, the question Roger saw Gwyn forming was "does he [Roger] love her too?" and the answer he realized was "Yes". There are few hints that Roger feels this way about his step-sister throughout the book (although the knowledge that he does allows you to reinterpret much of his behavior, especially how hard he was on Gwyn), and so Gwyn and presumably the reader, who's been confined mainly to Gwyn's thoughts and feelings the whole time, are surprised to learn it. Of course, by that point it's well established that the three kids are reenacting an ancient myth of two men who both love a woman, so the revelation might only be a half-surprise.
The real surprise is that it's Roger who is able to save her, rather than our hero Gwyn, and I personally LOVE that twist. Gwyn is already so bitter that he can't set aside his anger and hate for love. Roger can. It would be ridiculous to read the story as a simple allegory where the characters represent their "classes"-- rather, how Gwyn as an individual relates to his experience of his class is the background to his psychology. That we feel he is right to be so bitter makes it all the more tragic, and makes the message all the more poignant: however apparently justified it may be, hate will ruin you. Fans of the book should definitely read Garner's subsequent novel, 'Red Shift' which is an exploration of all the same themes.
I think it's very deliberately left unclear whether the curse is "finally over", or if a subsequent trio will find themselves in the same scenario here. The characters can't know, nor can the reader. That said, in the TV adaptation from 1969 which Garner wrote, there's a tiny dialogue-less epilogue added where you see three children (two boys and a girl, of course) playing around the Stone, and there's even a shot of them through the hole. I don't know if Garner wrote that scene in his script or whether it was his idea, but it suggests a continuation of the recurrence more than the book does. I think readers of 'Red Shift' will find it harder to accept that the recursion is over.