No manufacturer in their right mind is going to have open slots for delivery in the near future and not use them.
If you look at the features of 1.3, you will see that now, when there is a gap in production of a popular aircraft (within the time it takes to produce the aircraft), AI brokers buy the aircraft from that production slot.
If the slot is outside of the initial production window, a player can place an order. If nobody places the order, and the slot falls into the restricted window, again, the AI brokers will buy it.
There is however one area that could be improved. Suppose the production line is full, and your aircraft is spaced out, for example January, April, July. Later on, if production slots open up (due to bankruptcies etc), and suddenly there are open production slots in February, March, I think player should be able to place order for in those slots.
Or even better, again, if your orders are January, April, July, and production slots open up between those delivery dates, player should be able to move up his current orders, to slots that are "eligible". By eligible, I mean production slot that would exceed max delivery rate per player. This max delivery rate has been implemented for game-play purposes, and it definitely a great idea, to keep some balance in the game.
However, in the real world. That is not true. hell just look at American Airlines. Their current 737 order runs out in 2014 i believe. Yet this order for 200 737's start being delivered in 2013. If slots are open, then any airline should be able to get them no matter when their next aircraft is on order.
So, no, I would not agree with your other point of overlapping orders. It can happen in RL, but in AWS, it is not a good idea, since it would unbalance the game.