Yes, all correct (except a typo よしえさん).
"De wa/Ja arimasen" is more proper, but "ja nai desu" is not incorrect and actually used. (Some textbooks indeed say that "ja nai desu" is incorrect, though.)
The reason the textbook specifies to use "ja nai" would be that it's easy to make polite form (just adding "desu" at the end of the sentence).