GUC requires Y/B/M fare on KE to upgrade to Z or O depending on the fare. Award and low econ fares are not eligible, so it is a moot point as to applying the GUC to the BKK-ICN segment.

Just apply it on the ICN-DTW. If you misconnect due to KE's or DL's fault, DL will (should) accommodate you on the next flight on their metal and in D1, assuming there is a seat available. You should not need to re-clear the GUC if you get a good agent as they can force the override.
If you agree to get booked on KE metal, then it will be in economy and you can request the GUC be returned... same if they can only get you a PS seat on Delta metal, since you're already in PS.

Important thing is to get the ticket cleared/confirmed and reticketed in D1 ahead of time so that the fare class shows as OK and not the PS fare so if something happens they see you were already in D1 and not just PS but waitlisted for D1.