Dave, you are right: There is no point in opening up the hub so I will keep cool with this. ( I know from my small experience in mechanic that the perfect is the enemy of the good!)
Concerning my oil leaks, I got some improvements:
I removed the old sprocket, it was really dirty behind it so I cleaned it up;
After that, I installed it again (flipped) because the side of the sprocket whitch was on the hub side before was slightly marked and in fact, with the other side, after a big ride, the sprocket side of the hub kept clean (almost no oil)
I really need a new sprocket