There will more than likely be some kind of board level issue that would need to be found with test equipment like a multi meter and is more than likely a result of progressive corrosion caused by its recent swimming lesson, Seeing as its got worse over a period of time. When it was treated to the alcohol bath did all the shields on the boards get removed? Were the chips checked under?
Try this:- You can force Galaxy S7 or S7 edge to power off without unlocking the phone by following these steps: Step 1: Connect Galaxy S7 and S7 edge to wall charger with the USB cable and make sure it is charging. Step 2: Press and hold Volume down button and Power button (holding two buttons) for about 7 seconds. Step 3: Release power button, then volume down button. Now, Galaxy S7 or S7 edge should be powered off. You can wake up the screen to verify. It should show a charging battery icon.
I don't think its going to be a setting more likely to be a damaged wifi wire that often go thro the hinges and around the screen or the other thing it could be is a lose wire on the wifi card, think back did this start happening just after it got dropped/Knocked/Sat on/Wrestled with?