When it's time to order your car three months before you need it tell Motability that you'll be adding your carer to the insurance even though she lives over five miles away.
They won't refuse to add her to the insurance believe me. It goes against the whole ethos as to why Motability exists! Royal Sun Alliance won't refuse your request because it's not in their interest. They would them become a disabling barrier against you having independence. The insurance company is there to support you not cause disabling barriers.
You can't act at the moment as the scheme hasn't been rolled out. In other words even Motability will be in the dark about how it will operate!