Q: Why do Harpies and Medusa do not show up for defense in the simulator?
A: There are two divine units + envoys for each god. Two boxes on bottom left: 1st box for attacking player god and 2nd box for defending player god. Pick Hera in the defensive god box and Harpies and Medusa will show up on the defensive row.
Now you can run it through the simulator.
one Medusa is as good for defense as 26 Hoplites at a minimum. I just ran it in the simulator and here are the numbers: