Solution of CLRS question 4.6-2
The first point seems like a mistake. Find a different solution.

For the second point, $o(f(n))$ often refers to a function whose absolute value is $o(f(n))$. So $-o(f(n))$ and $o(f(n))$ are really the same thing. For example, you can write $n - 1 = n + o(n)$.

For the third point, here is a simpler example: $n + o(n) = \Theta(n)$. You can check that if $|f(n)|/n \to 0$ then $(n + f(n))/n \to 1$, and in particular $n + f(n) = \Theta(n)$.