You will have to start again with regard to the three year lease
Normally you would receive a pro rata refund of the advanced payment, but unsure of how it works when the advance is a grant. I think you would just have to apply for the grant again against whichever new car you want to order
The challenge will be regarding the length of rental if waiting a long time for a replacement car, however in the case of third party claims it will be the third party insurance who pays. However its not totally straightforward, you will be pushed to enter what’s called a credit hire arrangement with Enterprise, and there is a tiny risk that at some point you will have to justify a long hire period (as part of this arrangement you will also be asked whether you have the funds to hire a car yourself but again its just so they can provide justification of hiring a car especially for a long period and then claiming cost back from the third party). *You could google Credit Hire to find how it works. One positive about this is that you will get a like for like replacement (although if adaptions are fitted that is not quite so straightforward!). I had a large Mercedes to replace my Motability Audi.
Good luck but be prepared for some pain in getting things set up and agreed