Pixl frozen in hard reset, after message "BOOTING..."

Wow, that is extremely strange. At this point it really shouldn't be a hardware problem - it obviously works well enough to use Bluetooth, look at the buttons and control the LCD, so from there on it should be able to boot to Espruino just fine.

I just tried uploading exactly that firmware and it worked fine for me here...

Please could you try the upload again (and re-download the zip) just in case? And is it possible to send me a video of exactly what happens when you hold down the BTN1+BTN4 while applying power to the board?

On mine it looks like this if I boot it while holding the buttons:

DFU 2V16

RELEASE BTN1 FOR DFU
BTN1 TO BOOT

<=================================>
BOOTING...