Yes. Very good. There may be structures for which these are not appropriate, but a at first thought, it's good. . The class was so interesting. - similar to so interesting that I was on the edge of my seat. The class was very interesting. - not followed by anything else. . He is such an interesting man. - ok He is such a very interesting man. - ok He is a so interesting man. - not working so well. :(

Yes, saying "so interesting" is like saying, "That is so interesting." Saying "very interesting" is much more popular but it has the same meaning, "That is very interesting."

They are both correct. For example, you can casually say, "That's so interesting" when responding in general to something someone is explaining. You can also be specific and say, "The film was very interesting."

Yes, you can use or the other interchangeably.