If you booked this through a third party, it is the broker, not Hertz which owes you the difference between the rate you booked and the rate Hertz offered.

On the mileage issue, go ahead and get clear, concise and short statements from your seven witnesses. Make sure that the statements are just facts and make absolutely no conclusions. You can try sending them in. I give it a 5% chance, but why not.

The real lesson learned here is to never deal with third parties for airfare, hotels or car rentals. When everything goes right, this doesn't matter. But, when things go wrong, nobody takes responsibility. If you had booked through Hertz, you would have had a rate locked in and if it wasn't honored, a CC chargeback to fall back on.