We just booked on the M.S. Spitsbergen for Aug 17, 2020.
Per 1...Spoke with Hurtigruten and they said flight from OSLO leaves late evening. Return flight leaves around 3 am arrives OSLO very early morning. We plan to arrive a day early and book a hotel in OSLO then explore the day we arrive from the USA and check out some museums the day we travel to Longyearbyen
Per 2...I don't know about this trip for excursions except the one mentioned above to visit Pyramiden. On the boat they offer Kayaking, and also tours on their Polar Explorer boats (we found this worthwhile). Whenever you have a landing, weather permitting, you can take walks into areas marked out by the Explorer team which is usually about a mile long. One time on our Antarctica trip they actually did a hike which we did not do. They had snow shoeing, but am not certain they do this on this trip. One option that was available in Antarctica was a night camping on land. Warning...it was very expensive! They provide all you need, but you set up the tent. However, they were very helpful for us (not experienced with this)
Per 3...Come August you get midnight sun, but it is low on the horizon and sets August 23 for the first time in Longyearbyen.
If you check the link below and go to the part on Svalbard, there should be good information for you.
Lastly, we are doing this trip because we had such a great time on our Antarctica trip and had it recommended to us buy another couple on our trip. The crew and the Explorer team are terrific. The food was good to excellent with fresh baked bread each day. They also had an outside barbeque with steaks one night.
Just for fun, I included on of the pictures (not taken by me) from our trip.