For the orders that aren't sent to IB, you are saying there is never any client_OrderStatus update for them?
yes, that is correct.
So you would recommend that the plugin consider the orders cancelled if it doesn't get an order status update for an order it submitted within a specific amount of time? How long?
Yes. IB told me that the response is within 1 second but I would design for 10 or 15 seconds just to be on the safe side.