A. What model DN you got?
1. You can get 9 speed freewheels. May require respacing the rear hub a bit, but cheap and usually easy.
https://www.universalcycles.com/shop...&category=1665
2. 170mm is the nearest common size for plus/fat bikes. You really need to remove the rear wheel to get a good measurement.
3. Some bikes come with solid axles and freehubs, so make sure it's freewheel and not freehub.