It might work to get away from the collumns and rows in the message board and go with something similar to Ancestry.com. Each reply to a thread is placed under the message it is responding to with a tab and RE:. There can be several replies to the same message. This would use Cascading Sheet Styles and Nodes. Example:
Laura Bow is great
RE: My favorite scene is
RE: I like the part when
RE: Well I like this part
Plus or minus buttons could be added to expand to see the actual message.