Oh my. She's an adult. You have to let go if you want her to learn how to make good decisions. Taking away her phone was treating her like a child. Which then causes her to want to prove even more that she's not, causing her perhaps to make more poor decisions.
The more you fight against her choices the more she's going to stay with them. I suggest that you back off and let her learn from her experiences. By doing this now she may learn enough to not get married. The consequences are serious but still less serious than if you keep pushing and she gets married to prove her point. i.e. that she can do what she wants.
Taking away the phone pits you against her instead of creating a co-operative attitude. At 18 and living away from home she needs to be given the room to make choices without fear of retaliation from you. Instead of telling her what to do, keep an open mind, and discuss possibilities with her. Tell her, and mean it, that you realize that she's an adult and has to make this decision for herself. Let go of you making artificial consequences. Let her learn from the natural consequences of the decisions she makes.
Yes, the natural consequences are big at this age. That is why we need to allow them to make mistakes while they're still living at home. I wonder what would've happened if you'd not been hard on her for her lack of communication when she broke up with him? And let her work out the relationship knowing that you'd accept whatever she decided. Did you push her into his arms?
My daughter was living with her boyfriend at 19 and became pregnant. I've lived with this sort of behavior. I tried to convince her that she was making poor decisions and it only drove a wedge in our relationship. When she became pregnant, I told her she was now following an adult path and would have to be "on her own." I helped her find an apartment and get set up in housekeeping. Later I learned that she resented that I'd set her up the way I wanted instead of allowing her to set her apartment up the way she wanted. I thought I was being helpful.
Our relationship was quite rocky for several years because I kept trying to influence her decision making. She made some poor decisions but also some good ones. She did what she wanted to do. My involvement did not help her make good decisions. Eventually I accepted that she is an adult and has to make her own decisions and suffer the consequences without fearing my judgement.
She's 30 now, has a happy family, and is doing well. We now have an adult to adult relationship which has made all the difference in how well we get along. She now asks me for my opinion from time to time. I'm proud of her and who she's become. I realize that my backing off on influencing her life made a positive difference.
We have to let go of our children as painful as it is. They will form their life based on what they've learned as the grew up. Once they're an adult they are in charge of their lives. The more we try to make them do it our way, the more likely they are to do the opposite.
In regards to supporting our children financially. Yes, it makes sense that if we're supporting them they have to live by our rules. However, in the long run that is counter productive. My daughter and her boyfriend had jobs and were going to a community college. I helped. financially, because they were doing what they could do to support themselves. They had to follow my rules while living in my house but once they were on their own my financial help came with no strings attached.
The purpose of my continuing to provide some financial support was to allow them to learn how to manage money and live independently. For me, helping my daughter learn and grow takes precedence over my being in control.
I suggest that your daughter is on the right path. Good for her for getting a job and buying her own phone. Try to emphasize the good decisions that she makes while allowing her the experience of making poor decisions.