This is a wonderful story because it expands so much of what we know from canon, filling in and colouring JKR's explanations of Petunia and Vernon's attitudes and behaviour. That ending is quite chilling, but quite right, too, because it was always clear that they destroyed Dudley through misguided love.
I can absolutely understand her actions with regards to the owl, although at the sane time it's a dreadful thing to do. The impetuousness you give Lily makes her a perfect partner for James.
I felt sympathy for Mrs Evans but at the same time I wanted to shake her and make her look at what she was doing to her eldest daughter. You made me feel glad to see Vernon. Which is quite a feat. But he really was exactly was Petunia needed.