Does the start relay on the firewall pull up by sound?
If yes the starter or it's cables are open.
If no, the start circuit from the ignition switch through the trans range selector to the start relay winding is open. Only fuse in this line is #21.
What can happen with the battery backwards is if the start circuit pulls up, the starter would try to run in reverse under a lockup conditon. It is a DC motor and is polarity sensitive.
The huge current could open an already poor cable.
There is not much to the circuit that a voltmeter check would not uncover aside from the starter cables being an issue.
The mega fuse has no part in the starting except as a convient connection point on the route between the alternator and the battery for charging.
Only possible reason the mega fuse would blow is from the reversed power from the battery shorting to ground through the alternator diodes.
Then you would need to replace the alternator if it does not charge after the motor cranks and runs.
Good luck.