I think it's hard to see how you are going to win this one.
You put it in 'legal' and I think the legal answer is that you can ask your employer to change the term for you from having a CC to having an allowance but the employer can reasonably request that this applies only at the change of lease. I remember when I was a younger HR manager, we used to have this problem when someone (with a car) resigned and the trouble is the leasing company won't have the car back (or effectively change the company the same as to termination to get-out). We used to insist that new employee entitled to company cars used up the lease on a secondhand one first.
Your situation is even more demanding - you haven't left yet and you want to give the car back.
Unless you can persuade an incoming employee (informally) to take the car off the employer, or identify to the employer an incoming employee who might have the car (you will proabaly have to negotiate a bit on this - i.e. accept less allowance for the rest of your tenure), I think you are on a hiding-to-nothing.