that which you said “have to contend with a lot more prishus & minhagim” is not exactly accurate. Most Chasidim stress importance in enjoying mundane things in life more than Litvish. If someone is not successful in enjoying learning Torah or grasping it then Chassidus offers an alternative that makes that person feel good about himself.
I might agree with you in general that this is not something that should break a shiduch. Another thing to keep in mind is people develop and change especially with relationships and marriage. Its not just two statues that are put together in one house. A husband should be a guide for the wife if the husband is a talmid chachom on whatever level he is IE he learns, understands his mehalech from his learning and avodah, learned from his rebbeim. I don’t think that means that a husband forces her follow a specific set of rules especially nowadays. Husband and wife should be there for one another not for the presentation they make to others.
That being said we unfortunately have other external factors to keep in mind. communities and schools have very rigid and sometimes senseless criteria of who they accept and who not. The school that the husband would want for his son in this case probably wouldn’t accept him because the mother doesn’t cover the hair. the school that would accept them probably wouldn’t have such a serious learning program. until the kids come it wouldn’t matter so much but afterwards it might complicate things a little bit. They didn’t meet yet and aren’t yet crazy about each other just happened to notice each other. They should take this into consideration.