Is it helpful to perform cpr on a patient with an mi that is still breathing?

Probably not. A patient who is still breathing implies a functional heart rhythm, which is the primary reason to do mechanical cpr. So if one is still breathing, then supportive care to ensure a good, open airway, and calling for help asap, should be the priorities. If the patient stops breathing and no pulse is detected, and no AED device is readily available, then CPR needs to be instituted.
It depends. If the patient does not have a pulse, then CPR can be life saving; remember that for layperson CPR we really advocate fast chest compressions (100 per minute) in order to ensure adequate perfusion to vital organs until an AED or paramedics are available with an external defibrillator.
Yes. If a patient has a mi, it is possible that their heart is too weak to pump enough blood for survival. If the patient has no pulse-even if breathing-, then CPR is life-saving as you become their heartbeat. Mi with adequate circulation doesn't. Breathing brings oxygen, circulation transports it. Learning AED use is also helpful. Everybody should take a class in case their loved ones need it someday.