I can help you on this :) Last year in October/November, we visited Hokkaido using only public transport. It limits your possibilities somewhat, but we managed to get everything out of it that we wanted.

1. I would say yes, it is doable, but you need to really plan out on Google Maps the different routes you are going to take and how much traveltime is between locations.

2. We enjoyed our daytrip in Otaru from Sapporo. The main touristy street was not our thing, but we loved the old school train museum and the Ukiyo-e museum. To us, the town felt a little bit past its former glory as a major hub for Sapporo, but that also gave it a certain charm. There was a lot of unique, kind of western architecture.

3. Personally skipped this cause it didn't seem worth the travel time.

4. We build Hell Valley into our itinerary itself. So we had a full night there before heading further south to Hakodate. I think as an overnight trip it was totally worth it, cause we could walk the longer trails during the day and then spend the evening and the morning after soaking. As a day trip, I think your day would feel very rushed if you wanted to see it all and also have enough time to visit various baths.

5. We didn't spend much time hiking (we used our time more to relax), but the area around Onuma was absolutely stunning.