IMHO, that still doesn't rule out a battery fault if the second battery you've tested with has first been through a washing machine, then out of use for a couple of months.

If I understand correctly, the battery contains protection circuits to prevent over-charging or discharging, and these are mission critical as a Li-Ion treated wrongly will catch fire. So, the phone checks for this circuitry and refuses to use the battery if not present and correct; that would certainly explain a refusal to charge even when there's enough juice to light up a backlight. If there's any way you can test it with a borrowed battery from a live N900, do. This is still my prime suspect. I can't be certain enough to say "buy a new battery" - but it sounds like your battery has given up the ghost and I have little faith in the old one as a test...

I don't envisage that the brand of your charger makes any difference if it charges the phone during normal operation, but, confirm it's capable of 1000mA, not all USB chargers are.