Consider that on a good day under ideal conditions, the stepper motor is going to max out at about 900 RPM reliably
Not necessarily true.

Depends greatly on the size of the motor, it's inductance, and power supply voltage. I've heard of people with small mills getting a reliable 2500rpm from steppers. But the bigger the motor, the slower it's going to go.


2) In practice I've noticed that even with PPR set to 200 I'm nowhere near reaching 60000 RPM. Is the driver that actually limits the maximum frequency when I select a low PPR value to avoid mechanical issues or is the Software (MACH3 for example) that limits the maximum frequency?

The Kernel frequency in Mach3 will limit the maximum pulses, and the Velocity setting will also limit the motor speed.

4) I have read that torque change based on the PPR that I set but my datasheet only give the torque curve for 200 PPR. Is there a way to calculate (via matlab or something similar) the torque curve for each PPR setting? If I ask to the manufacturer to give me this torque curves is likely thai I will respond (manufacturer is STEPPERONLINE)?
Torque curves are meaningless unless it's the exact same motor, drive, and power supply voltage. And almost all I've ever seen are at full step or half step.