I think it's fair to say that invoice pricing is an imaginary number. Car salesmen will disagree and say that is actually the amount they paid for the vehicle, and I think they are correct. Dealerships do actually pay the invoice price for the vehicle. But, that is only a small snapshot of the picture. They get holdback which now everybody is also aware of, but they also get incentives on total number of vehicles sold, number of specific models sold, etc.. That's why they can sell under invoice and even under invoice and plus holdback. Because they're getting hundreds of thousands of dollars in bonuses from FCA for selling so many vehicles during whatever time period (monthly, quarterly, etc..).
The internet age has moved most dealers beyond pretending that MSRP is a real number, and they are now trying to rely on invoice. "Give us 2% over invoice so we make a little money on this deal." It's true on one hand - if that's the only vehicle they sold that month. But when that's their 500, or 5,000 vehicle sold that month they aren't just making the 2%, they're making that plus what they get for selling 500 or 5,000 vehicles.
As far as ordering a vehicle - incentives are the same, you can usually lock in the current incentives at the time of order and if they increase you can get those extra incentives between order and delivery. But, what you want are the dealer discounts on orders. Dealers are more motivated to move vehicles on their lot than on orders, as you've correctly noticed. Some high volume dealerships don't care. They move so many vehicles that they don't have many sitting on their lot that they have to pay "rent" for, so it doesn't matter if its on the lot or an order - neither is sitting long enough to cost them more money. But that's probably not true for your local dealer, they're going to give better discounts for a vehicle they already purchased that has been sitting for a few months that is now costing them money.
