I have no problem at all with the themes and scenes in the final book - to me, it doesn't matter if parts of the story have parallels with religion/Lord of the Rings/Narnia/WWII. What does matter is that these things made the story interesting, built up the atmosphere and kept you reading.
Who cares if JKR doesn't write beautiful, classical prose? I doubt that what she was thinking of when she wrote the first book - a children's story to be enjoyed by children of the 90's. What counts is the fact that she tells a good story and people enjoy reading it - that's what made it popular in the first place.
To be honest, probably the only classical literature she studied at university would have been French literature, as Classics doesn't involve reading Shakespeare, Jane Austen or Oscar Wilde. Now had she done her degree in English, then maybe you could argue that she was classically trained.