Well, I went to the guy who sold me the new battery, and replaced it with an almost fully charged new battery, and now the device is working AND charging the battery as usual
BatteryGraph & HeathCheck both says charging at ~650mAh... This behavior is like a riddle to me... I uninstalled the power-kernel, don't wanna mess with it for now, and also bought a separate universal battery charger.

Is it possible that my N900 "killed" two batteries beyond the point of being able to recharge? is there any "point" like that?

You know what - I never even thought of this option... Could that be a virus, that prevents the phone from recharging? I thought the N900's power consumption driver is closed by Nokia...