Why is "they wrote a story and later a book" wrong. Later is marked read for a reason.

Is there a difference between потом and тогда? Are they interchangeable?

The new voice pronounces потом as "patAm" when in the sentence, and as "pOtam" alone. The correct pronunciation is "patOm".

Is there any reason why it can not be "they wrote the story and then the book" ?

Does Russian use 3rd person plural to discuss an individual (that's already been established in context) like English does?

This sentence reads like it's one person in English.