Viking ... will be predominantly Americans. Americans are their market. You will meet people from all over the United States but few fellow passengers from other countries. Many of the people will be repeat customers. Consider that a good sign.
The free excursions may be walking tours of a couple hours. They give you a good overview of history, architecture and major sites.
Depending on the journey ... some of the side-trip tours meet the ship up/downstream from your port. I did not find this to be a problem. If a side-trip was over lunch, Viking Excursion included a served (not boxed) lunch. We never paid for a lunch while on an excursion. If you elect to not take an excursion but spend the day in town, lunch is on you. Depending on the port, ANY river ship may be a long walk. In Vienna it was easily 6 blocks to a subway. Then, a subway ride to city center.
Every port is different.
On excursions Viking did a good job of curating the Guide companies and the guides. Some were downright delightful!
OP, some people are happy to stay on board. Mobility issues would not be any better or worse than other mode of touring. For some excursions it will be a difficulty. Others, no so much.
One other note. Several of the other ships I saw had their dining rooms on the first deck. Viking dining was one level higher. As we passed one ship we were looking down on them. :)
Food ..... was good. Some people complained. Others praised it. For us, the "American menu" was the worst of the food. the free Wine selections were good. A good paid Wine list. At the bar, 10 different Single Malts.