The drivers are highly trained and experienced, and even guests who typically get motion sick are pleasantly surprised how they don't in the vans customized for this route (a benefit of taking a tour). In addition, doing the loop route means you are not coming back the more windy way you took to Hana; the backside return is much more straight.
To minimize, you should also try to sit near the front and perhaps consider taking medicine (such as Dramamine, or Hana Tonic that can be found at ABC Stores on the island).
Seating is not assigned and passengers are usually picked up at different times and locations depending on the final makeup of the tour that isn't known until the day before, so seating requests cannot be accommodated and seating is first-come, first-served. However, feel free to make a note at checkout so the tour is aware.