Greetings, NFA, and welcome to the forums.
I understand completely the idea that saying "I love you" is not frivolous. It means a serious commitment. Sometimes people can see it said for meaningless reasons on TV, like "I love this hamburger" and lose track of what it really means. So it is understandable but also frustrating when this type of disconnect happens. Maybe we need more words in the English language to help out.
Clearly she still cares for you a lot. She calls and texts you. It sounds like her friend is unduly influencing her. Which is also hard.
It's a shame if she's never lived alone that she's now living with another couple. How can she build her own strength if she always has someone else watching over her? The fact that she also isn't pursuing her dreams is a shame. She could easily get stuck in a rut and never move forward.
I wouldn't look at it as being a second choice. She is probably under a lot of pressure and caved when it got to be too much. You are her first choice but she's confused.
It looks like she'll probably never live on her own. Which is sad but a fact to accept. So I would offer to have her move in with you, if you're ready for that. It would be better for the child to settle sooner rather than later into a potential school system and stop the hopping around. It's hard enough on a kid to move through pre-school situations but to move from actual elementary school to elementary school is really rough on them.
I would start to present your case why your situation would be better, and why it's best for the child to start to find stability. I wouldn't be meek here. I would show her you care, show her you're willing, and show her that together you can do this.