It's not "a lot does happen to get that point," it's more like they ended up together only in the latest chapter. Just a few words are used to show that. It was the most horrible part of the novel.
I became alarmed when, after the tanks' construction part, the story started to rapidly increase its speed. The author quickly scratched lots of plot lines and tried to finish the story.
As a Nightingale's fun, I feel that he has neglected her. Her power (that was very promising) evolved unsatisfactorily. When Anna thought to allow her to be together with Roland, he just went into a coma. And after many events, in the latest chapter author was like "hey, btw Nightingale and Roland are together now."
There was no marrying scene either. Just some speculation that when Roland married Anna, Nightingale was beside them in her invisible form.