I have spent FAR too long on this crossword. I am in a similar place as you Barretter. I still can't see where the Letter I is discarded. Is there any significance in the initial RIDDLE discard?
If it's any use I have ASCI not ASTI, the C originally coming from the C / R clash with RAY.
I'm glad I don't send these in these crosswords as I would have no confidence with this one!