As far as keeping the coach as is... for me...its that the team plays more true to their real counter parts. They will run the type of plays (i.e. the bills) are running. I know that using the existing coach...you have more knowlege of the playbook as most are learned. I think Clay is letting the OC/DC call the plays...but for me, I want more hands on..so I call them.
You can use a created coach, but for me...it seems that the teams become good too fast. Just look at some of the post. Lions 14 - 1, Finns ..undefeated. Maybe this will slow the greatness down a notch or so. Again, we are doing this...to see how the games play out vs the created coach. Also, don't know why, but the CPU is running better against me. Maybe it has a better history of the real coach, vs created coach...not sure.
As far as the 3-4...I think you can change it from your coaching screen...I know I have seen it somewhere. Maybe it was where you pick your playbooks...I can't remember. I don't think you can tell when you hire a coach what they bring. Sorry I couldn't help more.