What's the difference between P840 and P852? 

840 turns the drive on and 852 starts rotation is that right?

Yes 840 switches on the drive.

p852 enables the pulses of the motor module's IGBTs.

p1142 enables the setpoint. Without that the drive would not get a software setpoint but could be switched on already.

So your statement is quite correct somehow  - of course the drive starts to turn when you enable p852, but in the end it just enables the pulses. p1142 is more about enabling the rotation software wise.