seeing <- Bluetooth messages at bootom of display in B2 F/W 2.10.223

I ran load("bootupdate.js") , nothing reported on the console but the watch just displayed 'Reloading ....' for 3 minutes until I did a long reset. I repeated on my other B2 and got the same result.

So did it actually fix the problem after that? I forgot to mention but the Reloading... is expected - as soon as that's shown you can just long-press to restart. bootupdate.js isn't meant to be called directly - it's used internally and would usually restart loading the app at that point.

The .boot0 you posted looks ok - I loaded it onto a watch and I don't see any messages - however the Uncaught SyntaxError: Got [ERASED] expected ',' message would definitely cause the problems you're seeing.

It's a particularly odd one as the part of the file that's shown isn't what gets uploaded when you update the bootloader. It's what's in the file when it updates after bootupdate.js gets called - but if that's the case it is hard to see why it would be corrupted