1. Are you sure you have the X96 Mini? The X96 Mini we've always sold doesn't have a power button on the device.
Regardless, for many android boxes, there should be an option in the settings to set the "power key definition" or something of that nature. I don't have one at hand so I can't provide precise guidance at this point, but it shouldn't be too difficult to poke around.
Note that Android boxes often have two types of settings menus; there's the generic Android one that you'd see on all mobile devices, and then there's the box-specific one that usually appears in a menu that slides out from the side. Usually both are mapped in the list of apps for the device.
2. I would chalk this down to the box itself being iffy. If the card works fine when formatted as an external card, then there's a good chance the problem isn't with the card.
BUT. Try a smaller microSD card first (somewhere in the realm of 8-128GB) and see what happens. Many cheaper devices can behave strangely with larger microSD cards (you see this sometimes with some retro gaming handhelds).
Regarding the box being slow - the X96 Mini is a budget model through-and-through. Good for understanding how Android boxes work, or as a pinch-hitter gift for a relative, but I wouldn't use one as my personal device (in the same way that wouldn't buy a £30 mobile phone to use as my daily driver). You might find a more mid-range model like the
DroiX X3 to provide a better experience.