If you’re using the chalk paint on the back of a hutch or something you don’t need to seal it but if it is on a surface where things will be sitting on it I would use a wax. https://lifeonsummerhill.com/how-to-paint-with-chalk-paint/
Yes, if you want it to be durable. You can use wax or waterbased polyurethane.
Yes, you do need to seal chalk paint to protect it from being scratched or absorbing any liquids. Use a wax or a poly. Most chalk brand brands carry their own brand of clear sealer or wax.
Here is a good guide I wrote all about chalk painting.. https://bushraslifestyle.com/how-to-chalk-paint-furniture-plus-chalk-paint-recipe/
Good luck and let me know if you do have any further questions, I always recommend sealing for the long lasting results.
It depends on the look you’re going for. If you don’t want it to look antique and chippy, I would seal it