1) Yes, but if you put the paper plates in whole, they will take a long time to break down.

2) I wouldn't. I don't feed anything salty. Salt is a preservative. That means that it kills bacteria. Even without the salt, grains in large amounts can result in a lot of mold in the bin. In small amounts, grains are OK.

3) You can add food scraps whenever you want. The key is not to have too much food in the bin at any one time. Worms thrive on neglect, but don't mind being doted on.

Remember slug/snails and salt? For that reason alone I won't put anything salty in a worm bin.
Paperplates are manufactured in such a way that it won't absorb liquid immediately. So, it will help the break down if you rip the plates in smaller pieces then moisture can go in easier from the ripps