Sorry for the hiccup, but the advice from the ladies is very sound.
Don't try to solve everything in one setting - the books are very clear on this front. Instead, stop it, accept what gains have been made, and move on to another subject (knowing you may have to physically leave in order to "stop" the current conversation).
This is another example as to why the reconciliation stage is so difficult. You are naturally excited to be where you are w/another shot at things and naturally you'd like to clean it up as soon as you can and get moving forward.
Just don't try to "build all of Rome in one day."
I know that advice "sucks" and isn't what we'd all like to hear, but patience and baby steps are what's needed now. BF has come back, but he's full of guilt and is still depressed. He needs time to work his way out of his hole...and you need time for yourself too.
He's inching along while you'd like to see him sprinting. It is the old "walk before run" thing, so give him his little chunks of progress. The entire wall will eventually come down if you keep taking little pieces out of it.
As for your EA, do whatever you can to answer his questions and reassure him of your committment to him. It is obvious it still bothers him, so make sure you continue to be up-front (as you have been) when he brings it up.
It is easy to get frustrated w/his repeated discussings of the whole EA, but it takes real love to allow him to ask and for you to continue to calmly and openly answer his questions...even if you've answered them over and over again.
In books on affairs, they say that the one who "cheated" will have to live w/the insecurities of the one who was "cheated on" for a long, long while and possibly be faced w/the occasional insecurity for the rest of your time together. He's showing his insecurity over your EA now, so steel yourself to "prove" your commitment to him.
Now, he'll have his own dance w/the "insecurity Devil" soon enough over his issue w/Helen, but that will come in time. I'm not sure he can handle his own insecurities about your EA and his actions w/Helen at the same time. You are the better judge of that, so keep watching for signs that it is ok or not ok to bring up Helen and him.
