No, it is not back-to-back. It is end-on-end, which is usually allowed by the fare rules.

Don't worry about flying back to DTW in the middle of the first roundtrip. Your trip is really MSN to SEA to NRT; NRT back to SEA back to MSN, an end-on-end. Connection cities are irrelevant unless you drop segments to turn a connection city into a destination (hidden city ticketing).