Usage of Nikko Pass via Kinugawa

Can I use the Nikko Pass to go to Tobu Nikko Station (from Asakusa) and return to Asakusa via Kinugawa Onsen Station?

Yes, Nikko and Kinugawa are both in the "free area" of the pass, where you can use local Tobu trains as often as you wish. See a map of the coverage area here:
https://www.japan-guide.com/e/e2358_002.html