Q: Can I use my guitar to help me when I transcribe? Secondly, based on what I have read in the Ear Training One Note Complete book, and to the best of my understanding, I will say that I am given the I IV V I cadence to give my ear a sense of key. When I hear the note, I am listening for what that particular note sounds like against the cadence, or the key, and I am ultimately trying to memorize its sound.
A: You are correct that the cadence gives your ear a sense of key. Using your guitar isn't a bad idea but you will need to improve your note recognition on the instrument before it will be of much use. To help improve your note recognition on the guitar I would recommend the Music Theory Workbook for Guitar Volume One, Music Theory Workbook for Guitar Volume Two and Single String Studies for Guitar Volume One. I realize you have some Music Theory and Reading background but you need to transfer this information to the guitar fretboard and these books will do that for you.
Keep in mind that the ear training book's exercises will take quite a bit of time before you are transcribing Chet Atkins chord solos. You will need to be very proficient in 2 note ear training before you will be able to start to transcribe in this sort of situation. I should mention that many of the chords that Chet uses can be found in the Chord Workbook for Guitar Volume Two. You should at least learn and apply these chords to the guitar so when your transcription ability improves you will have the physical technique to play these sorts of chord melodies.