First of all here’s the link to the service manual so you can read it:
https://support.hp.com/nz-en/product/hp-elitebook-8470p-notebook-pc/5212907/manuals
Meanings of lights pages 21.
* Power Light - going off means computer is off or has gone into '''hibernation'''.
* AC adapter/Battery Light - White: the computer is connected to external
* power and the battery is charged from 90 to 99 % and Amber with adapter means battery is charged 0 - 90%
What do the lights do if the AC adapter is unplugged and you try to turn it on?
What do the lights do if the AC adapter is plugged with the battery removed and you try to turn it on?
I am hoping “Typically I can get it working by removing everything (ram, battery, cmos battery) . . .” you did not remove the components in the order you listed above. Adapter, then battery always first thing.
Did you remove battery and hard drive and try turning on with just the adapter connected?
Did you take out both RAM modules and then try one at a time in each slot?
Let us know how it goes.