The class feature says that the effect applies to the arrow attack. So, I believe that they would have used the word "replace" if the intent was to replace the normal damage of the arrow. For reference, here below the relevant part of the class feature (emphasis mine):
Once per turn when you fire an arrow from a shortbow or longbow as part of the Attack action, you can apply one of your Arcane Shot options to that arrow.
So, I believe that they would have used the word "replace" if the intent was to replace the normal damage of the arrow.
This. The general rule is that when you hit with a weapon attack, you deal the damage listed for that weapon plus the relevant ability modifier. There's nothing in the Arcane Shot rules that say otherwise, so the general rule still applies.
One caveat: you can't apply the damage bonus from the Sharpshooter feat to the Arcane Shots that use saving throws. That feature of Sharpshooter requires an attack roll and scoring a hit; Seeking and Piercing Arrow have neither.