There are more litvish girls who switch over hashkafically to a chassidishe hashkafa than you would think. I personally know a few and have friends that have other friends who did as well. Each has different reasons for doing it, some more hashkafic, some having to do partially with the lower level of kollel support expected… each to their own on that part.
Usually, they switch over when its time for shidduchim and have no problem getting a shidduch. Going to chassidishe high school at age 14 in order to get a shidduch sounds a bit weird to me. I think that people should not raise their children for “a shidduch”. Instead, they should raise them as children.
I guess if your daughter is thinking of switching directions at a younger age, I am sure a chassidishe summer camp may agree to take her in, it would give a more social atmosphere without four years of commitment to a high school.