Hi! I also do EALD and I've never found anything useful for that here, so I will try my best to help you out, because our questions are very similar and I was just wondering about that a while ago.
First of all, subtleties in that context is how the author has demonstrated how subtle something about your personal, culture and social identity can be. It's all the subtle things you do, because of where and how you were raised and what you want to be, so the author might write about how he loves a certain type of food, but that's because he remembers being young and upset and his mom cooking it for him. Complexities would be his analysis to his subtleties (I do this and it's subtle, but why? Is it because of my culture? Where am I right now? Or who am I?)
I just read a little bit about the book, but all the subtleties that Billy does leads to his complexity of going down a destructive path, so all the subtle traits before the epiphany happens. All the truancy, hanging out with the wrong friends, just experimenting with illicit drugs and alcohol, etc.
How would I structure your essay would:
(Have a table underlining the complexities and subtleties of each issue that is going to be discussed)
1st paragraph: Personal
- Adapting to changed circumstances
2nd paragraph: Cultural
- Prevailing assumptions and beliefs about culture
3rd paragraph: Social
- Impact individuals' or communities' sense of identity on shaping the character.
Language - English techniques all the way AND what are the effects of these English techniques. Are they there to make the reader upset? Shocked? Surprised? Happy? What are they trying to convey?
Basically, what the question and rubric wants you to do is to how the book is used to communicate IDEAS, VALUES, INFORMATION and ATTITUDES, but you also can't forget to link it to shaping individuals' or communities'
I hope I helped a bit. x