To answer the microphone, thats the audio ic that went bad, in fact the phone i am using right now has that very issue. Also, the iphone 7s are notorious for baseband issues with cellular. What model is your iphone 7? (for example, mine is A1660)
First off, rice just doesnt do anything at all. It gets stuck in the ports, and under and in things. When water gets in a screen, it makes it look a lot brighter. You may need to replace the screen, but while your at it you should replace the power button too.