That's only true for voltage driven steppers with a high rated Voltage. For current drivers like the DRV8825 the resistance is nearly irrelevant.
That's true, but is only possible because of the inductance of the motor coils. The inductance and the applied voltage ( and the motor load ) determine how fast the rated current will be reached. With high inductance, low voltages and high steprates the current will not be reached within a steptime ( and this leads to less torque ).
If you connect two steppers in parallel, the driver cannot regulate the current in each coil, but only the total current. How the current is divided between the coils depends on the characteristics of the two motor coils and the load, and it is not determined. Only when the coils are connected in series is it ensured that the same current flows through both. But then a higher supply voltage is needed to achieve the same current rise ( and thus the same torque ).
With different loads, the motors will impact each other. I would never recommend this. The DRV8825 is a relatively cheap component, so you should always provide a separate driver for each motor.