Check the underside of the logic board for liquid damage, and clean with 95%+ isopropyl if there is any corrosion. Sometimes on the 12" Retina (your model), liquid can get under this, usually around the CPU area.
Check the gold pins on both sides of the ribbon which goes from the trackpad to the logic board (curly one). If there is corrosion, this will be burnt, so it will need replacing. Also, check both sides of the cable which goes from the trackpad to the keyboard. It's very likely either the cable(s) or the trackpad PCB is bad if both the keyboard and trackpad aren't working, since the keyboard is plugged in through the trackpad.
If the keys are sticky, the keyboard will likely need replacing too.
Regarding the microphone, check for liquid damage around the small headphone jack board where this plugs in. Sometimes the microphone cable can be burnt, or the board is bad.
To be honest, I would recommend taking it to a repair company which has the testing parts available, otherwise you will find yourself spending a lot of money on parts to test. Try out what I said first though, to see if anything stands out :)