Is there other bold in the volume? Could you just use the bold indicators (and, if you feel it necessary, do a tn on the tn page explaining to the reader that bold material is text dialogue).
On the page you sent, it looks like using bold would work…but it will depend on what is in the rest of the book 🙂
There are a couple of ways you could deal with the larger sections…they could be treated as displayed material (with blank lines before and after and adjusted margins), you could use emphasis …or do them as lists (with blank lines before and after and 1-3 margins) as they are a type of dialogue. If you are pretty far into the book and have been doing boxes, I’m not sure you need to change them. I would probably not use boxes, but that doesn’t mean you can’t :).