Thanks for all your valuable informations! I didn't know that there was a trailhead above the onsen. Let me check, and if I'm not so sure, I'll ask you again.
I went to Karasawa Valley last year, during GW (4/27~). I put the tent there and stayed for a few days. There was a lot stable snow, stable but getting wet at mid-day in the lower parts. There are quite a lot of very nice slopes there!
Another place that looks nice too is from Yarigatake-sanso. I never tried skiing from there yet, but both slopes (Nagano side & Gifu side) looks really great! Have you ever ski there?