Hi
If I understand correctly, I don't think you really need to query multiple collections. "messages" is a collection, so you can query messages and get all messages from your community regardless of what board or category they are in unless you specify otherwise.
This messages (khoros.com) lists all the fields and constraints for messages.
You can use the constraint conversation.style messages (khoros.com) to select the message type.
You can use the constraint conversation.last_posting_activity_time messages (khoros.com) to get questions that were edited or replied to in your 6 hour time frame. I don't think this would include accepting a solution though. I'm not sure if there is a way to do that. Also, you need to specify depth=0
So your LiQL query would look something like this
SELECT subject, id, [or whatever fields you want] FROM messages WHERE depth = 0 AND conversation.style = 'forum' AND conversation.last_posting_activity_time < [your time]
