BangleJS 2 unable to update firmware

It has had "BL 2v24" across the top at the start, which is presumably the existing firmware.

it is version of bootloader that is built as part of full build from source but there are typically no or very few changes in bootloader code itself between Espruino versions so it is OK to have older BL version than Espruino main firmware.

"BANK0 INVALID" error

espruino bootloader is based on standard Nordic DFU bootloader with added update from storage file so it has two separate ways of update. the bank0 invalid is message from Nordic BLE DFU update and says that valid flag (which is typically set after succesful BLE DFU firmware update) is not set.

The update from storage file done by the update firmware app (or maybe the IDE tries it too?) does not touch this flag https://github.com/espruino/Espruino/blob/master/targets/nrf5x_dfu/flash.c#L310
However if your watch can't boot into normal firmware and never could, this method cannot be used and never was. So what you probably used from the IDE is the nordic DFU way that failed to send the update correctly so now you have the bank0 invalid message.

In this state I think the best way is to use Android phone with nrfConnect app and try to upload zip file downloaded to phone from https://www.espruino.com/Download

You enable the BLE DFU update mode by holding button until it reboots and you see the progress bar, then you release it in the middle and check for the DFUTarg device via nrfconnect, connect to it and use the DFU button that appears to upload the zip file.

If nordic update from your phone does not fix it then flashing full firmware over SWD probably will (= using charging cable connected to some SWD debugger dongle) but that is a bit complicated so if it never worked then returning it may be easier.