Dripping from the top of the fill valve during refill is normal.
If the level in the tank is dropping, it could either be the flapper, or the seal under the flush valve.
Two screws hold the flush valve in place. It can be pulled, the interior of the tank checked for smoothness and reinstalled.
Any elongated seat will fit the bowl of the Soiree. Comfort and looks? That's kind of a personal thing.
The seat on the Soiree is pretty flat.