The circuitry spacings are extremely tiny, you might not be able to see with the naked eye something bridging two circuits.
Try soaking the motherboard in a (special) motherboard cleaning solution.
"Alcohol" is not a good idea (tm) - too hygroscopic - it dries fast leaving any absorbed water behind to continue the damage.
Yes - if your going to switch things, switch the mother board between the two devices.
Screens are too expensive to replace.
Had soap or detergent been added to the washing machine before the Kindle was retrieved from its bath?
Some of those products are extremely corrosive.
This whole effort may be a waste of time if something like Tide was in the washing machine.