Grit is crushed granite. Works like teeth to help grind up food. Oyster shell is just that - oyster shell that is ground up to smaller pieces to provide calcium, however, it dissolves and does not act like teeth to help grind up food.
If your chickens only eat feed, then grit not needed...
Definitely agree with ALWAYS provide fresh plain water, even if you are providing additive water in a separate container. In addition, do not use a metal waterer or container with additives in the water. A metal waterer is ok for plain water.
Keep treats to a minimum. They need their feed...