First of all, I would be very mad too if the salesman told me I could have it and then the one I chose didn't. Now, I am not an expert on them, but I believe that you only get Sirius if you get the upgraded sun and sound package (which was worth it I think, the sound system is awesome!) and your salesman was either mis-informed or was telling you anything to sell you the car, either way I would be pissed.
The jack on the side of the navi screen when it is down is not an input jack, I am not really sure what it is, but it would be a pretty bad place for an input jack, I know in my outie, and I think any with the Navi, the AUX jack is on the back of the armrest at the bottom, but I am not sure if this is what you are asking about.
As far as the back of the MMCS (navi system) there are many pictures floating around, I know I have seem threads on just the MMCS, but I can not remember where. If all you want is a look at the back take a look at this thread,
Installing a Backup Camera it is about how to install a backup camera, but it shows a picture of the back of the system.
As far as the sirius goes, I don't think it would be worth it to me to have to use a little screen when they MMCS is capable of having sirius installed. I would complain a little more to the dealer, because that is just unacceptable, and if they won't do anything, and having sirius is that important to you, maybe you have to install it yourself, or buy the parts and have someone else do it, I am sure you could find a kit out there somewhere, I found
Sirius Install Kit but I think it is for a Raider, but I am sure that there is something out there somewhere.
As for any of the wiring, I am not sure about that, I don't do a lot of wiring, but maybe someone else can help you with that.
Hope everything works out, let us know what you did.