MM:
Number one its in Even Haezer, not Yoreh Dayah (we are not dealing if the girl is a traifah 🙂
Two, if you read the Beis Shmuel there, what he says there is a seperate chairem (“Hakehillos”, not “Geonim”) is only if the breakoff is not mutual, or “many people are asked”. Meaning that for no reason it should not be done unilaterally, but if one’s Rav tells them to break off (perhaps with a second opinion), the chairem does not apply.