re Checking 6-8 letter words as valid: Check with 'BOGGLE' implementers. They have to check for valid words 3-8 letters long. They may have simple word lists.
There may also be a free simple REST service somewhere which will tell you if a word is spelled right.
I did find that 'https://www.thefreedictionary.com/dox' will return "<meta http-equiv="STATUS-CODE" content="404">" in the <head> where as ".../dog/" will not.
BTW, My reasoning for the one time 'hint' entries for long words (when they are normally being validity checked) is the combination of there being fewer of them to chose from, and harder to come up with having the letters in them you're interested in; making it Much Harder and frustrated to deduce the correct word. Yet it is too easy without any word validity checking.