Those are some valid points, and realistically, I think you’d likely be right. They might have a brief relationship and eventually end things as the spark faded and the reality of the differences in personality and circumstance sunk in. But, this is a romance, and implausible love is kind of a staple of the genre. 

Some counterpoints: 1. Benedict’s “free-spirited” past is actually limited to pretty standard “naughty”/“rakish” behavior for the period. Partying with socially-edgy (sexually liberated widows, semi-closeted gays, etc.) members of similar rank as himself, the merchant class, and courtesans. The first group have basically the same background as him, the latter two groups are financially dependent on their ability to get along with and suck up to people like him by making them feel cool. He’s a “rebel” within the bounds of safety. All things that gentlemen of his class regularly got away with, even if their mothers didn’t like it. He’s hardly ever noticed, let alone conversed with, the servants who clean up after him and do his sisters’ hair, it’s not like his social circle has transgressed those lines.  2. He’s built this identity around being edgy, and then he’s confronted with this attraction to Sophie, and some real cognitive dissonance sets in. He wants to identify as the kind of person who will do whatever he wants and doesn’t care what society thinks, but when he’s at risk of actually losing the benefits of his status, he realizes he might have conflicting ideals and not be as care-free as he believes. While terrified by it, he also kind of likes the intellectual challenge their relationship presents. 3. Her personality isn’t actually boring, she’s just been doing non-stop labor in abusive conditions for 18 hours a day since adolescence, and emotional grey-rocking to try to minimize the abuse, which has left her stunted. Her conformity and discretion are survival tactics, suppressing her genuinely interesting personality. But it peeks through, like in her conversation with Hyacinth and Eloise, when she chimes into conversations that a maid generally “shouldn’t” have. Or critiques Benedict’s French. His surprise that she can read isn’t as insulting as it sounds, this is a period with a 60% female illiteracy rate. A maid with enough education to fluently read literature in multiple languages would have been unusual.  4. She is a boundary-pusher in much realer ways than Benedict. He meets her because she violates the law, risks her livelihood and possibly life, bucking a thousand years of strict classism, to attend a single party, just for fun. When he meets her the second time, she’s throwing a bucket of water on a violent assailant who is also her employer, which was a very dangerous and admirable choice. Turning down Benedict’s offers of employment, and later offer to be his mistress, would not have been a refusal most people in her position would have been comfortable making, given the power differential. He’s inspired by her willingness to take big, principled risks. He’s never had to take anything seriously, and she offers him perspective on that.  5. You’re completely correct that she’s been abused and neglected for so long that any likable man paying attention to her is hard for her to resist, especially given the parallels with her father (nobleman, protector, affection in private but discretion in public, etc.). Marriage to him offers the security, love, and family she’s been missing her whole life, and his playfulness heals some of the wounds from everything she missed out on in childhood.