Crispin didn't leave and wasn't fired either. They asked if he would do the sequel, but he priced himself out of it due to demanding too much money.
Yes him not returning was the reason why they killed him off and was also a reason why future George was upside down. This all gets explained in the special features on the disc.
Yes him not returning was the reason why they killed him off and was also a reason why future George was upside down. This all gets explained in the special features on the disc.