I've never really understood why it's okay for professors/instructors at colleges and universities to have close, personal relationships with students but it generally isn't well accepted if they are close, ROMANTIC relationships. I mean, does the fact that you love someone romantically impact your impartiality in professional arenas more than loving someone like a sibling/son/daughter/other family equivalent? Baffles me...
Like BP said, there are usually official channels to go through to have relationships acknowledged so that A could protect himself HOWEVER he is already married, so more than likely the college isn't going to accept it since it would not just be a student-teacher relationship, it would be an extramarital one (which, let's face it, most people are going to label an affair even if it isn't).
I also understand your desire to push through with it no matter what if A and/or E make a move, but you have to wonder if A is thinking clearly. If you are willing to rush into something saying "screw the consequences!" why wouldn't they? Crazy NRE sweeps everyone, so while he may be willing to risk professional credit right now, is he going to feel the same way in a year? Two years? Do you want to risk him having serious consequences to be with you and then regret it? You've got the right idea about talking, talking, and talking some more to make sure you're on the same page, but I just wanted to throw the possibility of him regretting it (not necessarily regretting YOU, but regretting impetuous action on a relationship with you) out there. Is that a hurt you are willing to risk?
Another think you may want to make sure they are aware of is you inexperience in dating as an adult. The less experience you have in serious, adult relationships the more likely you are to make mistakes. This isn't a bad thing, it's just the way it is. Experience brings the ability to cope with things more effectively. Luckily, you are reading up so you should avoid some of the mistakes or at least be able to handle them a bit easier!
There are a lot of issues here that you need to figure out for yourself, let them figure out for themselves, and then talk about and all of you figure out together! I wish you luck... I don't envy your position.