I'm a pretty old fashioned discipinarian as things go. I put my foot down on tantrums, defiance, aggression, or any other habits that would have gotten harder to nip after age 3 and would never be tolerated...my favorite simple, calm, loving, EFFECTIVE guide was "Back to Basics Discipline" by Janet Campbell Matson. That timeless system worked wonders with my three very different kids ranging from easy, to mega-stubborn, to super angry by nature.
But I have to say in the examples you give here..I don't see a problem. Kids do have feelings, and they can be sensitive and intense. They cry sometimes and get sad over stuff...the wrong book being read or wrong friend doing wrong thing..sometimes daycare providers need to apply "discipline" which isn't much in daycare settings..time-outs and whatnot. The trick is not to put too much attention on these normal types of little kid "problems" and adjustments. They need to work it out for themselves. Sounds like your preschool staff is doing fine.
As for making her not say "I don't want to", there is really nothing wrong with voicing the opinion that the child doesn't want to. The problem is when you say they must (if it's something they truly must comply with) and then they throw a big fit or start lashing out aggressively or whatever. You don't want those habits to start. But just saying they don't want to is fine as long as you keep the upper hand in a clear and calm manner.
So for an example, "Ok, sweet pea, time to brush teeth and go to bed." "I don't want to." "I know, but it's time for bed, so here we go." : Child complies. Yay! This type of thing can be achieved without rewards for doing what's acceptable in the Back to Basics Book. But if it's more like "OK Sweet Pea, time to brush teeth and go to bed." And the child says, "I don't want to." and you repeat your request and they dig in their heels and throw fits and run you through hoops and never comply: That's not OK. Get the book. If she was good up until now it means she's easy by nature and going through natural 3s. The book will work well and quickly like it did with my first who needed very little discipline to behave. My other two had me working much harder by 18 months :)
3 is the perfect time for her to learn all these things. You want heathy boundaries, not complete subservience.