In front of people - the audience will never know the difference. Your guitar buddy seems like he will not know the difference. So until your ear gets better what difference does it make?Malcolm..so you usually assume this progression?
I I I I
IV IV I I
V IV I I
I guess the chorus/bridge are a different story though. It just seems to out of reach for me. I don't have a good ear at all so that doesn't help. If I understand you correctly, assume that progression and if it doesn't sound right, try something else, like maybe the song has a quick IV for the second bar, or something like that? I see how that helps if I'm just playing with a cd but if I'm in front of people, if I don't know the song its not like I have to to rewind and replay something.
You are trying to make this more complicated than it really is. Chill out, you gotta start somewhere - try the best you can and enjoy the ride.
That progression you listed is a more like a 12 bar blues progression. Country is close to that, but, not as sophisticated. More like:
http://www.cowboylyrics.com/tabs/williams-hank/cold-cold-heart-616.html And then that same progression repeats in every verse and chorus. Country is dirt simple if you make it sophisticated you mess it up. That D7 in the third line, according to theory it should be a Dmaj7 chord, in Country you will not see a lot of maj7 chords just dirt simple dominant sevenths thrown in willie nellie. Grab hold and enjoy the ride.