What you said about Sebastian finding his salvation in religion got me thinking about his situation, and it's almost an ironic journey for him. He wanted to flee his family and his mother in particular, which also involves fleeing the control of religion in his life, but he ends up seeking solace in the arms of the church, and by extension his mother, family, childhood and past.
A return of the repressed for Sebastian, perhaps, which is how I view Julia's situation as well. No matter how much they try to escape their mother and family and religious and societal expectations, both Julia and Sebastian find themselves drawn back. Cordelia, who is the only one who appears to embrace her religion in a positive way (unlike Bridey who is essentially pretty passive), she manages to get away and find peace within her life without psychologically shutting anything out.
I agree with you about the Julia/Sebastian/Charles situation, but I think that part of what he loved about both Julia and Sebastian was what their family stood for - they both represent a way of life that he is both attracted to and appalled by. He admits that Sebastian was "the forerunner" and I think there was a sexual and romantic love in addition to a friendship and an admiration. It is all the more tragic that he couldn't forge a lasting relationship with either of them because of prejudices: society's prejudice against homosexuality (with Sebastian), and the religious prejudice against divorce and living in sin (with Julia).
What you say about the role of representation is fascinating: the idea of the frames being an alienating technique. Waugh published the novel in 1945, and so his original readers would have experienced the war first-hand, whether in action or at home. They also would have experienced it second-hand, through representations in art, through news reports, through stories told by others, and that makes me wonder if there was a sense of detachment, or experiencing the event through...