I need some help figuring out the 2 gang I have in a bathroom which consists of a light fixture and an exhaust fan.

within the two gang is 4 romex cables each one is a 2wire w/ground.

from left to right coming from the top of the box is romex for light, romex for fan, then two romex cables to the far right coming in.. one I've identified as from the breaker.. the other I have no idea.

This is an 8 yr old house and when I removed the switches the first thing that caught my eye was the nuetral (white) were wired together with some ground wires. i know this is wrong so the first thing I did was remove the switches and clean up all the wiring.. but when I reset the breaker nothing worked. so I began messing around with everything and my tester showed I was getting power up to the bulbs and everything. Nothing would work until I re-attached the ground from service to the neutral wires. So I reconnected as it was and put it back together..

I know this isn't good practice but I'm clueless as to why the neutral isn't working and how to track it down. any ideas?