When you said system settings are you referring to "System Preferences" or "System Report"? Have you have checked system report and battery information and USB/touchpad is not showing up?
Press and hold "D" key after you power on the mac to load Apple Diagnostics. Alternatively if you can connect to internet, press and hold "Option" + "D" key at the same time to load Apple Hardware Test.
Try this first. Disconnect your battery then disconnect your screen assembly. Then reconnect your screen assembly and reconnect your battery then try to power on.
when you hold the option key, did you see the arrow pointer but do not show any disks? If no means your keyboard maybe faulty. you can try both the left and right option key one at a time and see if you get a response. if not use an external apple usb keyboard. take note that MacBook air boot up time is quite quick so you must be holding the option key already when you press the power button otherwise you will lose that window to enter boot option menu.
1. Ensure logic board is completely dried up. Use hair blower with low temp setting if needed. 2. If above did not work, buy a keyboard replacement and see if it can trigger the power. Liquid spills can easily kill a keyboard. And power button is integrated in the keyboard circuit.
It is the issue with the internal SATA HDD flex cable connector. This Macbook Pro A1278 2012 models have this common problem. You need to buy a replacement part and change it.
I did a lot of bare screen replacement before for Macbook Air 11-inch and 13-inch for 2010 - 2012 models, meaning i only replaced the internal screen and re-used the housing and the cables which all work out quite okay. Question is for the newer models 2013 and above, do these use the same type of internal screen with same LCD connector as with the 2010-2012 models? I know that the iSight connector is different and wider but the wifi connectors and LCD connectors seems the same.
I did a lot of bare screen replacement before for Macbook Air 11-inch and 13-inch for 2010 - 2012 models, meaning i only replaced the internal screen and re-used the housing and the cables which all work out quite okay. Question is for the newer models 2013 and above, do these use the same type of internal screen with same LCD connector as with the 2010-2012 models? I know that the iSight connector is different and wider but the wifi connectors and LCD connectors seems the same.