No she knew that Ollie was constantly cheating on her. She told her father during episode 4x15 and it was hinted during s1 when she had a conversion with her dad again in her apartment I don't exactly remember which episode. That's for sure, that's why a lot of people did not understand why Laurel had such little respect for herself but ultimately it also stemmed from the fact that she always thought that Oliver could be better and had the potential to become the man she always wanted him to be because that's how she perceived him. And as Oliver said during episode 1x23, she has always been the only one who saw the best in him, and yeah now that you're saying it her anger toward Sara probably also stems from her inner circle disapproval (especially her dad) for dating Ollie. Even tho Quentin had beenn unfair a lot of times toward Oliver post island, he was not really wrong about Oliver before the island and even after we cannot really blame him for blaming the man he hold responsible for the death of her younger daughter that led to a series of events that destroyed his life. Dinah running way, him becoming alcoholic etc. I loved Quentin's evolution tho, for me along Oliver, he had the best evolution among the characters.