Another insurance guy here. When the policy automatically renews, the details should remain the same.
I would guess that as you spoke to an Indian call centre (and ours are as useless as most of them, but not my decision to employ them), I suspect they changed the driver rather than adding a new one.
Our calls have been kept after being recorded for about 5 years - however, it does not mean they have necessarily recorded every call.
I think Kira has unfortunately hit the nail on the head - when you asked them to add your mum, they should have sent you a new certificate of insurance with your mum on there.
Obviously the commen sense approach here is that you asked her to be added, and you would assume she was - not sure where that stands in the eyes of the law, but in that respect I think Ethel will be correct.
Hopefully they will find the call - I honestly cannot advise you how they would proceed if they cannot locate it. I suspect in that case Harry will be correct in that the insurers will go with whatever the notes on the system say - hopefully the note states that your mum was added, and they just failed to print the certificate/make the amendments
If the police were not involved in the accident (i.e. attended the scene and asked for documents to be produced), then don;t worry about prosecution. Obviously be aware that your mother MAY end up liable for any costs if it is decided that she was not insured. Not much help I know!