I went to the convent from 1967 until it closed in 1971/72. I believe the nuns transferred to the Convent of the Sacred Heart in Hastings, E Sussex, which still exists, and they might have archives...
http://www.sacred-heart.e-sussex.sch.uk/Deal library and the Maritime Museum also have some info and photos.
My father was also at the school in the 1930's, and remembers it being hit in the Blitz. I have fond memories of my time there; I think it still had boarders in my day. There were certainly strict nuns, but also kind ones who influenced my life and career.But I know how convents and churches were not always happy places.
The school was an integral part of the town, and it was sad when the school was closed down and demolished. It is now the site of a carpark and Somerfield supermarket. I personally think the event marked the start of heritage decline in Deal.
Your mother's story is very interesting, and very poignant. I'd love to know how you get on.
best wishes