Hi, Allan,
thanks a lot for this comprehensive answer.
As to the 'power' button, in other posts I read the action you describe (6 sec continuous push) is said to reset the keyboard, for example if you want to unpair it from a given computer. I also cannot confirm from my experience that the long press switches the keyboard off. The only way to securely switch it off is to turn off the Bluetooth connection on the computer (as the keyboard holds the connection even at a distance of 35+ feet) and/or loosen the screw that locks the battery compartment.
What also strikes me as odd is that, if I control-click on the Bluetooth symbol in the menu bar, and get to see the specifics of the Magic Keyboard it says that the software version on it is 0.50. So Apple never got around to issue at least a 1.0 of the OS of the keyboard? Weird. Even if there was an update, there is no conceivable way that I have found to update the keyboard's software. Somewhere I read that the update comes automatically with the OS update of your computer, but I guess that Apple has long stopped the support of this old piece of hardware.
Which is a shame, because mechanically speaking it is still one of the best keyboards around.
As to different batteries types, yes, quality not rechargeable batteries last definitely longer than rechargeable ones. But never more than two weeks. It really makes you wonder how much the technology must have improved in the past years ever since the Magic Keyboard came on the market, as even the worst of today's rechargable bluetooth keyboards hold their charges for months.
Best,
Michael