Al, your bullet list is spot-on correct. However, these items are accounted for in the full equation of motor power conversion. There are 2 terms in the equation that frequently get overlooked. Powerfactor is the obvious one, and it will vary depending on how much load the motor is sustaining. The efficiency factor is what is covered in your bullet list, and it will vary with the design of the motor.
The full equation is:
hp = I x V x pf x eff / 746
For most motors at nameplate loading, the powerfactor will be 0.80 to 0.85. For good quality motors, the efficiency will be 0.85 to 0.90. I have found that setting both of these to 0.85 gives fairly realistic values for most smaller motors.