Before we start, my textbook declares lgn as base two.
I only have one question, how did log2 become n (both highlighted in yellow in the picture)? Is it because log2 = 1, which is too small to even matter?
Since we're using base-$2$ logs, $\log2=1$, so $$n(\log n-\log 2) = n\big((\log n)-1\big)=n\log n -n\,.$$
All of the stages of the calculation are written as equalities, so nothing should be being discarded as too small to matter (and nothing is).