As I continue to struggle on this, I realize that it's probably not as much a problem with Import SQL as it is a generalized (but not universal) paste problem.

Import SQL is not the only paste that is having issues. When I copy text onto my clipboard and then open my model, I can paste that text into the table name textbox. And I can paste that text into a column name for a table. But when I try to paste the same text (same clipboard session) into the table comments textbox, it instead pastes a random table into my model (the same table that it was pasting into my model when I tried to import the DDL as explained in my original comment.)