Thanks again Mark. The final result is to keep my list a discussion one (of a sort). I have set it to moderate both members and non members. If someone posts to a non existent topic, I will simply post it to the correct topic or create one on their behalf assuming it is a valid message.
Default action to take when a non-member posts to the list AND Default action to take when a member posts to the list is MODERATE.
Announce only style the list attribute allow_list_posts is False and for the Discussion style, it's True.
I guess I should leave it as TRUE if I am keeping as discussion list, but since I am moderating all comments, I am guessing it doesn't matter. Am I right?
This is the setting in Postorius -> Settings -> Alter Messages -> Include the list post header an all it does is include or not include the List-Post: header in mail from the list.