About 4) 私がいたって彼がいうこと聞く訳ないじゃない。
If it is to mean, "Even if I warn/advise the person, they can't listen to me." what is "じゃない " doing in this case, reinforcing the idea, with a somewhat rethorical tune?
Also, why is it that everything seems unnatural? is this book poorly translated or am I just imagining things(due to my inexperience I guess)