Why don't you just go ahead and DIE DAD!!

What I heard during the country bumpkin part was "I was born sucking on rat guts" but that might just be me.

As for the roast beef/"just tickling" part, I think she was testing her parents to see what their reaction would be. The way they reacted, she could probably tell that they didn't really want to do anything about it, so she said that he was just tickling. Her spitting out the roast beef was just to show to her parents that she wasn't feeling well so she could get away from them.

I have no idea what age he's supposed to be, but there was a picture of him diving in high school in 1992, so 5 years later he could be anywhere between 19 and 24.

'Her spitting out the roast beef was just to show to her parents that she wasn't feeling well so she could get away from them.'

That's probably true... but personally I also thought it was meant to convey Devon's disgust at her parents and their lack of conviction in her initial statement that she is being mistreated.

Think about it. She's just told them that Brett is being abusive towards her. Their reaction is one of complete disbelief. Not in a 'oh, my God, that's awful' kind of way... instead in a 'I don't really think that's possible' kind of way. They don't believe Brett is capable of that because he is a rich boy who lives on their street. He is 100% "one of us" - unlike 'white trash' Trent. Brett also does not fit the antiquated notion of a pedophile (i.e. he's not a dirty old man in a raincoat).

Such a disbelieving reaction is unbelievably hurtful to a child who has suffered abuse / felt threatened by someone's inappropriate overtures to them.

Because Devon is perceptive, she immediately realises her parents are not going to side with her on this. They probably can't even comprehend it. So, like the above poster says, she is indeed 'testing' them. And they fail. Effectively giving up, she then changes the story to suit her mother's wishes ("He was just tickling me.")

However, if a terrible injustice has been done to you, most can still feel it in their core. I believe this feeling of anger and frustration builds up inside Devon and she expresses her disgust at her parents by spitting out her roast beef - the same way one would spit out a rotten morsel of food. Her spitting up is a rejection of what they have just done to her. Still feeling 'sick' (at them and at her situation), she then asks to be excused.

I don't know the answer to your last two questions.