Don't say you are happy etc, just attach copies of the documents.
Now, I am an active 80 year old but,I have to say, feeling rather old at the moment, so whilst you say, "elderly" it really needs more meat, so is this person partly or wholly immobile ? The more facts you present, the better able they are to accept rather than send out the usual Fob-Off letter, and if they ignore all your reps and documents, you have powerful evidence of their obduracy to present to an adjudicator.
Post your reps for review here if you want.
PS - The CEO just sees an unattended car parked in contravention, so serves a PCN to the car. He can't possible know the circumstances. It is their back office who receive reps and should, if they are a good council, cancel the PCN. However, I suspect they are under pressure to get the money in from council bosses, so send out tosh. Hopefully they won't do this to you.