Okay, I admit it, I just don't get it (spoilers)

Here is my best guess to your questions.

Lark knew that Rollo would have to accept the promotion, it was too good of a opportunity. She realized the Pelham was in love with her, so she would have lost his love if she married Rollo. And if Rollo and Lark married it would ruin the relationship between the brothers. She also knew that she couldn't wait the five years. Therefore, it was hopeless with Rollo so she jumped at the chance and chose the Italian. You also could see that Selina was doing everything she possibly could do to make Lark's life miserable and that she would do anything to prevent her brother from marrying Lark. So the note "Selina is right" is her acknowledging that she agreed with Selina about Rollo accepting the promotion.

I have a slightly different interpretation: I think that the deciding factor for her was when Selina told her that Rollo had left the house never to come back. Which was obviously a lie, he left to clear his head and think, most likely, because he returned a few hours later having made his decision to marry Lark. But thinking that Rollo had already made his decision and left her, Lark was faced with living in the house with Selina who loathed her and Pelham who was going to be disappointed because she loved Rollo not him, waiting for Rollo to come back when she thinks he didn't even love her enough to put her ahead of his career. Her only other option is to marry the Italian and leave England. So she chose the lesser of the two evils and married the Italian guy. She said "Selina is right" in the letter because she didn't want Rollo to feel bad about what she think is his decision to do exactly what Selina wanted him to do.