I have paid a lease in full before only because it was a company car and you don't pay the interest over the 3 years so I saved a decent amount in the long run for the company.
Let me describe your scenario with an analogy.
What you have done is actually placed a $20,000 bet. However, the payout is only $20,000 so even if you win the bet you just get your money back. No house in Vegas would ever try to get away with a game of chance like that because they know it is completely and totally unfair and deceptive against you.
Honestly, there is nothing O.K. with prepaying an entire lease. It is simply a $20,000+ bet with no upside because you just get your money back and a huge downside in that you could lose your entire $20,000+.
If I was you I would go into the dealer and say that you changed your mind and want to buy the car out. Try to negotiate the buyout down from the residual value. If they won’t budge then just cut your losses and take a low interest loan out for the residual amount.
What this will do is switch it from a lease to a loan. Now your insurance company will pay off the entire value of the car instead of just paying of the residual amount if it ever gets totaled or stolen. Honestly, the situation you are in right now is just simply too big of a risk.