I think this is a questionable plan. There is no guarantee that the I-751 will be approved without an interview......and there is no guarantee the I-751 will be approved during a combo interview. Therefore, you could have to return to the US for biometrics, then an I-751 interview, and later an N-400 interview.
Unless you plan to be outside the US for longer than a year, why bother with a re-entry permit? Afterall you have said "So we are out of the country for two months in the spring and two months in the fall as we visit and stay with her family in South Korea while we are in Asia."