Depolarisation of the atria leads to a low amplitude ‘P’ wave, which indicates atrial contraction and is displayed as a positive deflection on an ECG monitor.

 

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Yes, the P wave corresponds to atrial depolarization.

 

Additionally, the so-called "QRS complex" is mainly ventricular depolarization (along with atrial repolarization, which is masked). The T wave represents ventricular repolarization.

Refox,

 

Normally, yes. But the amplitude and deflection are subject to change from individual to individual. The deflection isn't necessarily "low" nor is it always positive. It can deflect negatively due to damage from MI or other injury which causes an abnormal pathway through the atria on it's way to the AV node. Misplacement of electrodes will also show a negative deflection though normally you'll see that show up in the QRS as well.