This is the GM lease guide.
The easy way is the dealer writes the lease up to end at the normal mileage.
The hard way is for the dealer to do the retroactive $0.20 per mile.
Make sure you are clear and the contract lease agreement is clear.
As for the failure to disclose it as a loaner, I would not hesitate to lease it, but they need to make it right since you are paying for a vehicle you can not drive. Can they “loan” it to you while you wait?