Adverbs in Paragraph

Is this paragraph correct:

"A 99-year-old man from Italy was going through a dresser drawer, and he found letters his wife had written and stored. In these letters were notes between his wife and another man, someone she’d had an affair with 70 years PRIOR .

Despite his being 99, and his wife being 96, he PROMPTLY filed for divorce."

a) I'm particularly interested in knowing if the use of 'prior' is correct.

b) While at this, which sounds better: 'promptly' or 'immediately'?