water splash caused minor iPad Power Button problems, what is the risk of mainboard corrosion?
I've been to one more repair shop, they also refused to open it and redirected me to official Apple Service center. I scheduled an appointment at the Genius Bar.

Apple representative provided me with following information:


They don't open iPads. Assuming you have Apple Care and there is a problem, they replace the whole unit.
They don't advise opening it, they say the device is sealed and glued and it would be hard to put it back together as it was.
They say they understand my concern about corrosion, after my requested, they showed me more or less where motherboard and batteries are placed.


I returned home and did more research on my own.


I checked settings of my iPad and got it's exact model: A1584
I Googled "A1584 teardown"
On this website I found very detailed photos of the inside. I grabbed the case my iPad had on during splash - it still had the mark after the splash. 
I downloaded the x-ray photo and highlighted the pattern of the splash on it, I also highlighted where mainboard is placed and with arrows I marked potential entry points of water.
I estimate, since the device was not submerged, only trace amounts of liquid could enter inside of the case and I think it's reasonable to say it couldn't reach the battery or mainboard.
I estimate worst case scenario, potential corrosion could damage the the smaller circuit boards connecting power button, camera and volume buttons to the mainboard.


Reference on iPad teardown

My decision:

Taking into account following information:


Device was not submerged and I estimate liquid couldn't reach critical components.
Device is sealed and non-trivial to open, after closing it again might become less hermetic.


I decide to


Not open it for cleaning of the motherboard.
Prepare extra means of precaution for such events, purchased a case with better sealing.