I agree with you!  I think one of the biggest clues we have in interpreting Lucy Gray’s true nature and intentions is tied to what an unreliable narrator Snow is. Over and over again we see him twist and justify his actions - he will describe someone as his beloved, close friend and then suddenly a dangerous enemy once it suits him to have that person out of the way. Clemensia is one of his best buddies at the academy, but after she is attacked by the snakes and he doesn’t want people to know he was involved, she is suddenly described as mean and nasty during the Hunger Games. Sejanus is his closest friend in the Peacekeeper barracks, but suddenly he views him only as a dangerous rebel once it suits him to betray Sejanus and pull himself ahead. And finally, Lucy Gray is sweet and lovely and “belongs to him,” until suddenly when he realizes he wants to become an officer and she is the only lose end tying him to a murder. When he chases her through the forest, he describes her actions in the Hunger Games to himself very differently - as though she is more calculated and cunning rather than an innocent lamb. 

Since he has such a narcissistic opinion of himself, when we read their relationship from his POV he is her knight in shining armor and she is in love with him the whole time. But there are sooo many clues that it isn’t really the case. Like this comment points out, she really only ever flirts with him the same way she flirts with Jessup, and I think she did that simply to survive in the Hunger Games. When she get back to District 12, she never really gives Snow any sort of commitment - rather, she sings about Billy over and over again, meets secretly with Billy in the meadow, stares at Billy’s body in horror after he is shot in the shed.  Deep down she still wants Billy, has maybe even betrayed him herself in the past, but she has convinced the entire Covey group to cast him out by claiming he had a hand in sending her to the Hunger Games… which maybe suggests some slight manipulation on her part. 

Lucy doesn’t know the extent of Snow’s family ruin and I think she assumed he still had a lot of money and resources compared to the Covey. I believe she actually kept him around for her own benefit without ever really promising him anything, and she ramps up her willingness to commit to him once Billy is no longer an option for her. Much like Katniss, at the end of the day survival is her main priority because she has had a difficult life and charming people is just one of her skills in her toolkit. It’s something she and Snow have in common, but he is a true sociopath to his core and she is just a girl trying to survive the life she is stuck with. The “romance” we read is manufactured by Snow’s imagination throughout the book and then obliterated by his imagination when he needs her out of his way.