I picked up on the comment from an earlier thread - one of the ic's is mounted on the reverse of the board because of track length! At the frequencies involved the difference in track length is not going to be an issue. If you were really worried about track length you would not be using through hole components! Put the ic on the right side of the board! I think you're being a little paranoid of the things that aren't an issue and neglecting things that are!

Shoving the 0v track around your optocouplers is really only serving to increase the capacitance between the cpu side and the real world side - separation is the key here! Keep the real world stuff on one side of the optos and the cpu on the other with as much distance as possible. You've also got the 0V fill getting close to the SS relay outputs - maximise your distance here. The 0v fill is to keep down EMI radiation from the board- especially the micro - it is not a cure-all! Use it where needed - keep it away from items connected to the real world! You're using optos for isolation - let them do the job! A high voltage spike on the real world side couples into the cpu side via capacitance - seek to minimise this capacitance and you will minimise the coupling.