The initial merge can either win or lose the duel for you. When Urchin started training me, he could have pummeled me off of every merge, and if not that, he'd at least be on my 6. Basically, just listen to Urchin. If his advice works for me, it'll work for you

And as for the MA... every time someone attempts to HO me, I'll merge with them, (provided it's not too hot in the area) whether they like it or not. I think it's only happened on 2 occasions that the other person merged as well, but they were both regulars in the DA. All you have to do is nose down and fly under them, and pull up as soon as you're right underneath. The pilot in question will probably be flying inverted spraying bullets at you, and won't be able to recover in time to keep you off their 6. That's the most common "merge" I encounter in the MA. The other type of "merge" is where the other guy doesn't invert for the shot... he simply noses down at me and hopes to hit me. These people invariably just zoom past after they miss me, and since I'm usually in a G10 or a dora, they don't get very far.
Doing this I have never been hit on the initial "merge," and have only died once... to one of the duelers, of course (forget who it was). I plan to make a few recordings for success with things like that in the MA.