I don't like show Margaery even when people think she was kind to Sansa all time. I think she was softly manipulating in contrast with her grandma and Cersei manipulation to Sansa. Yes she didn't stop meeting her after Sansa marriage with Tyrion announced but she also told Sansa to enjoy her marriage and intimacy to a man whose family killed Sansa's. It's easy to think that she is genuine but she is smart enough to know that this advice is meaningless. Marg was never genuine to Sansa.
I'm a bit more ambivalent about her. I think it's possible that she manipulates Sansa to learn more about Joffrey and yet doesn't wish her evil. These are not necessarily mutually exclusive?
As for Margaery's advice? They left out some of the Tyrells. In the books it's Garland Tyrell who tells Sansa she might be happy with the 'dwarf' or at least happier than she would have been with Loras.
I think the Tyrells are all like that. They look after their interest but apart from Olenna they are not actively working against people who are not enemies. They planned to wed Sansa to Willas for her claim and do that under the guise of helping her (and it would have helped Sansa to escape the Lannisters) but when it comes down to it, it's not about helping her but how useful she is. They are not monsters. They are still nice to Sansa but they also don't go out of their way to really help her (or Garlan would have offered to help Sansa escape). Sansa is so used to the abuse of Joffrey's court that a kind word makes a huge difference.
I don't know if that means that the Tyrells are not 'genuine'. They never promised escape, just a marriage to Willas - which suited them. And when that didn't work out they were polite, even somewhat nice, but unhelpful. On the other hand they never promised anything else. So, they are neutral with a hint of benevolence as long as it doesn't go against their interest?
I don't especially like Margaery's advice but it's quite smoke to what her brother says in the books.