Froggy, jumping in late, looks like most of your questions were answered. One thing you mentioned about negotiating the selling price and the effect of residual value.
The residual value is calculated based on msrp. so regarless of what the selling price is, the residual should stay the same as a % of msrp. Example $40k msrp vehicle, 50% residual $20k. if your selling price was $40k, your amount to be amortized is $20k.
Or
$40k msrp, 50% residual, $20k. Your selling price is $35k, amortized amount $15k.