Fairly certain each letter is only used once and they appear to be added in order, ie. add A to 7 across and Z to 24 down and alphabetically in between, not sure what is meant by "added alternately". So for 14ac in the right hand you're looking for a four letter word, and an F, make an anagram from the letters, get a word that isn't defined.
Not 100% sure all that is correct as I don't have all that much done but I have answers around the beginning, end, and middle and it appears to be holding up.