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How practically difficult would it be to connect a BLE keyboard to the bangle?

I don't think it's too hard, and I would have done it but the issue is finding an actual Bluetooth Low Energy keyboard.

Why is the default unbuffered/direct if there are such benefits to double buffered?

Double buffered is less screen size, and the buffered examples (80x80,etc) use a bunch of RAM.

What is the refresh rate of the LCD?

120Hz? I'm not sure, you'd have to look at the datasheet. Realistically you will never hit it though because the bandwidth to the display itself is too low.