You forgot to include something in your question, so it can't be answered.
Acceleration is the rate of change of velocity aka how fast the velocity changes
Yes, acceleration can be positive and negative because acceleration is a vector. It has both direction and magnitude. The direction is what makes it positive or negative. Negative acceleration is...
In the rotating frame, it can be (though it doesn't have to be). In a non-rotating frame, no (though it can be uniform in magnitude).
a=dv/dt a=acceleration v=velocity t=time.