Great questions. Stacking is generally okay, though there are some caveats. The big one is where you’ll be stacking them. If it’s in a closet or cabinet with limited airflow and space, stacking might not be appropriate.
If you want to stack things, I’d recommend getting some footers between the Power Plant and everything else. Something like these would be effective without risking scratching the top of the P15: https://www.musicdirect.com/vibration-control/bdr-cones
You can plug your GoldenEar subwoofers directly into the P12 or P15, that would be no problem. As you said, it’s rarely running at max power. We ran the bass towers for the IRS Vs on P10s and the P10s never broke a sweat.
The features don’t change between the P12 and P15, though some folks have been in your shoes and compared the P12 vs the P15 in their own system and the consensus is that the P15 sounds better. The P15 has a more robust output stage and greater energy storage which leads to lower output impedance - this is the main reason for the performance improvement.
Either one will be a big step up over going straight from the wall or from a power conditioner, though.
I hope this helps out! If you have any follow-up questions, I’m happy to assist.