Sorry to hear about your situation. Here's how I see it:

1. You tried to book a discount prepaid rate, but the charge didn't go through. It's not clear to me how you booked that rate, but if it was on the website directly, my expectation on a prepaid rate would be that it would have authorized the charge before returning you a confirmation number, and that if something went south after that point, that somebody should have contacted you. I don't know if the same is true if you book on the phone with the property or SPG. In any event, you didn't actually prepay, so I can see the difficulty in getting the prepaid rate as a walk-up guest.

2. Regarding the authorization at check-in, authorizing for double the base room charges is pretty standard in the hotel industry. Not every hotel has this policy, but enough do that I'd expect it to happen unless you find out otherwise. It's for this reason that I first got a T&E charge card (Diners Club at the time, now Amex) as you can really burn through a credit card's credit limit quickly when traveling, between hotel and car rental pre-auths. It's a bit strange that they didn't seem to be able to authorize for less - they must have something hard-coded. At the very least, they should have been able to charge you for the exact cost of the room upon check-in and turned off the ability to incur any incidentals (which would include no PPV TV, no phone calls, and no room charges).