Diagnosing Water Damage - I/O Board?

My sister's macbook air was exposed to water while in it's carrying case.

I disassembled the machine and cleaned the logic board and I/O board with alcohol and a toothbrush. I only noticed slight water stains and corrosion on a few pins on the I/O board. After resetting PRAM and SMC, and trying to self-diagnose with AHT, I am still having the following issues:

The machine seems to function correctly until the operating system loads. The boot sequence displays the apple logo and loading bar, but once the OS turns on, there seems to be power issues and the screen/keyboard backlight turn on and off in a flickering cycle. The keyboard and trackpad only work when the flickering stops, however an external keyboard and mouse seem to work fine whether the screen and keyboard back light are on. The strange thing is, as soon as I select the "shut down" option from the OS, the flickering stops and both the display and the keyboard backlight remain fully lit with no problems.

What would cause the machine to malfunction only when the operating system is loading and using resources? Is it possible this is all related to the damaged I/O board? (I cleaned most of the corrosion off, but I imagine it's not 100% restored.)