Update: after careful examination I found a stain of lead colored grease on the baseboard below compute module.
After wiping it out I saw that the board underneath has a slightly brighter shade of green. Not sure if that's the mask reacted with that grease and lost its color or it's I accidentally damaged the board during transportation.
Now it boots (blue LED, coolpi logo, Ubuntu boots) only when holding down Loader key, green LED lights constantly and blank screen otherwise. Still looks like either a crack in the board or contacts misaligned/poor contact since mechanical pressure makes it boot.
Do you provide some kind of baseboard PCB layout or service manual, and do you sell baseboards separately? I will try to take laptop to local repair shop for diagnosis and update with the results.