It's weird that deck plans weren't available when you booked, because in my experience, they are made available at the time a ship goes on sale. And just based on the process, they would have to be available in order to book through the Princess website.
Either way, it does seem unlikely that you would be assigned a wheelchair stateroom if you don't need one, but it is not impossible. If you booked through a travel agent you'll need to go back to them for assistance.
As far as customer service being outsourced and terrible, get used to it. I have it on good authority that over the last year Princess has, and continues to, let go of quite a number of full time employees from various departments in favor of offshore contract employees.
Good luck!