Assuming for a moment that you're right, that your daugther's ex is the best type of relationship for her, I don't see how your plan will help in the long term. Let's say you manage to break things up with her new boyfriend and she goes back to her ex. Your daughter is only 18. It's only matter of time before some other guy comes along and sweeps her off her feet. If she's broken up with her boyfriend for another guy once, she'll just do it again. If he's really the perfect guy for her, the best thing that can happen right now is for her to be without him for a while. Then she might come to see how good she had it. She would have a chance to see what could go wrong in other relationships and to start missing her ex. Either she would take him back for good, or if it was too late and he'd moved on, she'd search until she found somebody else with the same good qualities. If she's only going back to him because she feels her other options were cut-off, then she won't appreciate him anyway. You're being very short sighted to think that breaking up your daughter and her new boyfriend will get you what you want.
I'll also give you a little warning story. My sister broke up with her first love after being together for a few years and got together right away with a new guy. My mom heard my sister's ex's side of the story after the break-up. Her heart got really caught up in feeling bad for the guy. She became obsessed with the idea that he was the sweetest guy, and that he and my sister should really have stayed together. Even though my sister recalls my mom complaining about the guy before, after the break-up he suddenly became the perfect guy for one of my mom's daugthers. My mom tried to get between my sister and her new boyfriend like you want to do. You know what happened as a result? My sister just moved out of my mom's house and into the same house where the new boyfriend was living. She also stopped talking to my mom for two years after this. For two full years, I had to watch my mom cry about how much she missed her other daughter. So be very careful with this one.
In my sister's case, the new boyfriend turned out to be a better guy for her. They are now married over twenty years later. The reason my sister had left the previous guy wasn't just because she'd fallen for somebody new but because there were real problems and she'd wanted to leave for a while. Even my mom no longer says my sister should have stayed with the ex now that she's seen how things turned out. My mom just wishes my sister had been kinder been to the ex during the break-up. The reason my sister was so cold to him, though, was because all her friends who'd been dumped warned her about leading him on. Their experience was that the dumpee always wants to interpret everything as a sign that the dumper will come back, and it only keeps the dumpee from moving on. So my sister was trying to do the ex a favor by keeping some distance from him.
In short, only time will tell who's right or wrong for your daughter. In the meantime, though, you need to try to keep your distance from this. I realize it's tough to do when they're both living with you. But it can only make things worse if you get involved.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy