Hi Adam,
The important thing to keep in mind here is that old people love a little trip out. Take your Mum on a few test drives and let here try climbing in and out! Let the sales reps make a fuss of her.
You might like to try the Mazda 5. It’s quite high but the sliding door will also give very easy access through a wide door. Nissan’s QashQai is much higher, but not 4x4 high (which might be as bad?) so that could be a winner for you. But you will need to wait a year for the prices to come down to your budget.