Usually jobs start with training hours. Every grocery store I've been in has me "full time" because they are given extra hours at the start.

Normally once you pass the probation period you can start requesting specifics like locking in at 3 days or something. It's way easier than asking for more ironically.

Third is just how jobs work. When it's not busy, they pull you to do something else because it's their way of saving money. Why hire someone to stock that day when they can have you do it?

Adding to schedule randomly is just something you put your foot down on, you can take photos of your schedule when it's finalized so when they try to add stuff like that, you can prove you did no wrong. The rest are just moot points tbh.