Thank you for your detailed investigation!
As for the question, I'm not very good at English, so it may be difficult to convey. sorry.
1. It's not common. Most Japanese eat white rice. Next, many people eat brown rice for their health. I didn't know multigrain rice. And I've never seen it sold.
2. This depends on the person, so it cannot be said unconditionally. Sushi is an image of eating when celebrating. However, Japanese people always have sushi as an option when going out to eat out. So sometimes I go to eat without any particular reason. Therefore, some people eat it often, and some people eat it only occasionally, but I think that many people eat it occasionally because it is expensive.
Sashimi is eaten more often than sushi. Because sashimi is always sold to supermarkets. And no technology is needed. However, it is expensive and I don't buy it often. That's why I eat it once in a while. In other words, many people eat sushi and sashimi when they want to, so they eat it occasionally. (Poor people can't eat much.)