It is most likely a defective part. Double check the connectors and make sure everything is tight. If any areas of the cable aren't making contact fully this can happen. If the connections are good then the part needs to be warrantied.
You could try a penetrating lubricant like WD-40. I don't think it would damage anything in the battery compartment as long as you aren't drowning the thing. Then just make sure to clean it up really well when you're done.
Before you attempt to power up a water damaged device you should always open, disconnect the power and properly clean all water from the device. After that is all done if this message is still showing up it could be the front camera flex cable or a few other things.
Have you tried logging into google on a PC and clearing it from there? It sounds like it may be stored in your google account history and not your phone. You should be able to do it from either place but sometimes it is easier on a full version of the webpage.
Some samsung devices have an app called my files that you can use to browse both the internal memory and the memory card, if it doesn't have that then you can always download a file manager app from the play store.
Your microphone port is probably plugged with lint. Check for a small hole in the bottom of the frame somewhere near the charge port and see if it looks like there is something inside it. Can they hear you if you put the phone on speaker?