I don't know what the cardiologist knows about you, but maybe he wants the more generic metoprolol so that if you have to change at some point, you can try the diltiazem for benefit. Many patients find that what they have taken for a few years begins to lose ground to the disorder, and then they are faced with more limited options. This is just a wild guess, unless something about your arrhythmia makes metoprolol a clearly better choice...finally.

As for the cardioversion, AF begets AF. A heart spending a long time in AF tends to be resistant to interventions of a kind, certainly the 'easiest and earliest' ones. This includes a cardioversion. Or so I thought. Then, my BIL, who was found to have been in AF for three or four years, as far as anyone can tell, was given a cardioversion last spring and his heart happily converted and has been in NSR ever since. Go figger!
I don't know how much you have learned about AF, but left too long it can cause poorly reversible changes to the myocardium, and even cause prolapse of the mitral valve. Eventually, some patients end up with heart failure. It could be that your cardiologist is now sufficiently worried about your brand of AF, it probably being long-term persistent at least, that he wants to do a last ditch attempt, via cardioversion, to see if you might be due some magic. And, he may feel that, this late in the game, an ablation might not help you because it would involved a horrendous amount of lesions in order to block the signal.
This is all conjecture. Again, I know next to nothing about you, and I'm very much just a curious layman.