Not a teacher.
This is my first time hearing (or, rather, reading) the word daffiness. I had to look it up, but my original guess seemed to be correct.
I am familiar with the word daftness, but only through my favorite Scottish YouTuber, who uses the words daft and dafty to describe himself or his subscribers quite often.
I personally can not cannot tell the difference between the two and it* is not commonly used here (but seems to be popular with the Scots!) no full stop here - the exclamation mark serves as the closing punctuation mark to the sentence