PH5: Mission: Langley Heist

Good morning, Mr. Hunt. We have quite a situation here. Your mission, Ethan, should you decide to accept it, is to descend from a ceiling DUCT via ladder and reach the list of covert agents on FILE.

Clues to the 17 four-letter words that are missing have been provided in alphabetical order of the answer word. Change one letter per rung. Never change the same letter position twice in a row.

This puzzle will not self-destruct in five seconds. Good luck, Ethan.