I wouldn't drain the oil with the engine idling and in gear, primarily due to the safety issues. I don't like being underneath vehicles that are running and in gear, but that is just me. It won't get most of the oil out, as you will still have what is in the torque converter, so at best you will get half of it out. I wouldn't overfill it on purpose, and I would make sure to fill it at the specified temperature (see the BMW official procedure).

I don't see the negative impact of letting the parking pawl take the weight of the vehicle. If you abuse it by letting it roll and then putting it in gear (worst case) you can break the pawl, which is a simple mechanical arm. There is no connection with the reverse gear IMO since the parking function is mechanical, and not hydraulic.

I don't think the transmission failures have much to do with the power being put through them, from reading of the common failures. I believe it has more to do with the electronics package, sensors, and so on, than mechanical failures. There have been issues with a clutch pack (good photos on this site of a rebuild), but the lack of a consistent failure mode suggests to me that there isn't a single weak point. If the ZF transmission was a weak point, why do just as many failures happen with the GM transmission? The common elements are the controls package, and the cooling system.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue