Thanks for the post.
i don't know how to turn the automatic sorting off...I'm not sure it's really "on" unless you've turned it on through a portal relationship or in a script. FMP stores the last sorted order (I think).
But I do know how to manage a task list since I do it daily.
Put a number field per task record (mine is called "Priority Level" ) and sort by that. If you want to re-prioritize, just change the number. If the date changes, it won't affect the sort order.
I include a layout for myself which is a list so that if I want to do a full renumbering I can just tab through the "Priority Level" feilds. If you just want to change things around a little bit, you can change 1 , 2 , 3 , 4 , 5 to 1 , 2 , 3 , 3.5 , 4 and reorder. Key thing is to sort by something you have full control over. As you get the tasks done, all the pesky decimals (if any) will self-delete as your task records get archived.
Hope this works for you!