My first thought is that you are giving too many warnings. Ask any of the kids in my daycare how many times I ask them to listen and they will tell you "One". For the throwing sand thing I would have given one reminder to stop and if she continued she would have a time out. If after time out she continues disobeying she would go straight back to time out. If she has to sit there and do nothing while the other kids play then that is her choice.
If she doesn't sit properly at the table I would be putting her in a booster seat with the seatbelt on. If have this problem with my own 2.5 year old daughter, I warn her that if she doesn't sit up properly then she goes in the booster seat - which she doesn't like.
Can you rearrange your daycare so that things she shouldn't be getting into cannot be accessible to her? Babygates to block areas that are off limits? Door knob covers? I hang all my daycare kids' backpacks on hooks mounted on the back wall of my coat closet that has doors. That kind of stuff may make your day abit easier.
Can she have nap time/quiet time somewhere else so that she can't disturb the other kids? Do you ensure that your mornings are busy and active to increase the chances that she will nap?
For the potty training, if the parents aren't working on it at home then it is going to be hard for you. Have you discussed potty training with her mom? Is this something they want right now? If she is leaking through her clothes then she needs to be changed more often or likely needs a bigger size diaper.
But if the mom isn't being supportive of you then maybe terminating care or doing a probationary period is in order. You need to stand your ground on the things you need, have a talk with the mom and tell her what you need and why.