Right, so then we're back at my initial question: Is difficulty irrelevant as long as you know the net hash and your own hash rate?
Sounds to me that it is irrelevant. I mean, the difficulty by itself doesn't mean anything if you don't know net hash and your own. But if you know the net hash and your own, the difficulty number is irrelevant.
for bitcoin maybe, but there are coins where, simply knowing the net can't make you know your income, you need to knwo also the diff, i think it was the case with pascalcoin
however when the diff adjust you may find that you will earn less for that time because the diff still need to catch the nethash, this happen with a slow diff retarget like the oen with bitcoin...