You could use the car for work because your income would benifit your wife. Volunteering has no benifit for your wife. If you were to call the shops to get food on your way home, you could argue it is for her benifit!
Motability clearly state
Your vehicle needs to be used by the disabled customer or for their benefit.
This does not mean they need to be in the vehicle for every journey.
It means other drivers in the same home can use the vehicle for:
Shopping
Visiting family
Other routine activities
As long as the disabled customer benefits.
We take misuse of your car very seriously. This means things like:
Not using it for the disabled person’s benefitYou or someone else driving without insurance or while you’re banned
Using it in a criminal act
Lending, sub-leasing or selling it
Using it for business reasons we haven’t agreed with you, like as a taxi or for deliveries
Not taking good care of it
If we find you’re misusing your car, we might terminate your lease, stop you from getting another Scheme vehicle, and send you the bill for any repairs or estimated loss in its sale value.