re: "If we are to take the situation as described in the Amarna letters as proving that there was no Exodus/Conquest in the 1300s how can we avoid understanding the documents surrounding Egyptian/Hittite dealings - Kadesh etc., - as proving that there was no Exodus/Conquest in the 1200s either?"
Simple: The Exodus occurred in neither period but in an earlier period.
re: "If we are to take the situation as described in the Amarna letters as proving that there was no Exodus/Conquest in the 1300s how can we avoid understanding the documents surrounding Egyptian/Hittite dealings - Kadesh etc., - as proving that there was no Exodus/Conquest in the 1200s either?"
Simple: The Exodus occurred in neither period but in an earlier period.