On the A320 RTO, our policy is on setting of the park brake the Capt would tell the cabin to remain seated the FO would carry out ECAM actions OR emergency checklist for a fire on ground ignoreing the ECAM. The reason for the latter is the transition from master 2 off on Ecam actions you have to go to the checklist and start from the top again. This way you start from the top and read actions only once and saves the confusion and time.
As for the APU for take off, well there are many thoughts on this, basically it is another engine that can fail or go on fire so we opted to leave same off for take off.
Hope that helps somewhat.
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Gravity always wins....