I understand that after being yellow for a while, it still won't boot after removing the charger.

Does it try, does the LED fade in when you hold power? Voltage in Japan doesn't matter, the power supply is switching.

At this point, it's either the charger (least likely, since USB doesn't do it), the battery (likely) or charging circuit.

If I were you I'd get another BL-5J from another phone (Xpress music 5800, e.g.) and try to boot it up. If it does (and likely will), try to charge that battery. If it charges, it's the battery. If it doesn't, Nokia Care Centre.

I'm leaning towards the battery. Find someone to lend you a battery for 10 minutes. Swap them, use LSHAL to get voltage, let it charge for 10-20 minutes, then unplug and compare.

If you can't find anyone to help, buy a battery. Worst case scenario, you have a backup battery. Store it in a bag with a bit of rice or silica gel, in the fridge, approx 50 percent charged. Shelf life in these circumstances is about 10 years, you won't lose it.

It certainly looks like a dead battery, but it could be other things, a poor contact in the USB charger socket, burned charging circuit, etc.

Don't think it's software related, but you said you can't flash.

If you can't boot up several times in a row, it's bricked and needs to be taken to Nokia. If it no longer does anything, even with USB flash mode, you likely need to take it to Nokia. Try a battery swap if you know someone, but if it does nothing any more, it's time for service.