I've been installing Audiovox units for around 12 years now, working for a car-dealer expeditor, and I know the 9233 very well.
1: It has a single port for the shock sensor, and the shock sensor is included with the unit. There are no other sensor ports.
A good installer can splice additional sensor(s) into the shock sensor wiring if he knows how, but the 9233N also offers a couple of hardwired inputs for trunk opening, and I think maybe one more.
So at any rate, to answer your question more simply: There's only one plug-in sensor port, which will already be occupied by the shock sensor, but a competent installer could add more.
2: Installing dealer "cannot" add the accessory? That's weird. Maybe they don't stock it or don't want to order it, as it's not a hot seller it seems.
The part in question is the ASCL2, also called CarLink. There is an older ASCL1, but it does not have GPS tracking.
Installation isn't difficult, but Audiovox publishes horrendously written literature that neither the professional nor amateur can understand. So that part would certainly slow you down.
The ASCL2 simply plugs into the 9233, with two more wires you would have to connect to power and ground on your own. (Yes, they were too stupid to design the thing to just get power/ground where it plugs into the alarm.)
The ASCL2 comes with one year of basic service, which allows the user to operate the remote starter (which it sound like you won't have), and lock and unlock. You'll also get a text message on your phone if the alarm is triggered.
(The 9233 has outputs to lock and unlock the car, but the installer wouldn't normally connect them; it's not hard to do, but would probably not be free.)
The ASCL2 does have a GPS tracking feature, at an additional cost of $50 per year.
Keep in mind that you could choose any type of GPS tracker you want, it just wouldn't integrate with the alarm like the ASCL2 would.
By the way, you can find Audiovox manuals online if you dig hard enough. techservices.audiovox.com and sign up for a free account. Their search feature is so crappy it doesn't recognize hyphens. Search for PRO9233N and you should find it. There's an older 9232 that's very similar, so if you find that, it's close enough for you to get a general idea.
You'll cringe when you see how wonderfully their documentation is written.
3: Audiovox, Viper, and Python all make good products. I would be more interested in knowing about the installing dealer's reputation, quality of work, and warranty.
4: I've heard different opinions. I think many installers are no longer hooking up the starter kill on immobilizer-equipped cars. If you want it hooked up, might not be a bad idea to say so, just to be sure. The starter kill relay is included with the alarm, and it doesn't take long to connect; you shouldn't be charged any extra for this.
So, the two sides of the story are A: The car is theoretically already impossible to steal, but also B: The starter kill could keep people with access to the keys from stealing the car (children, house burglars who found your keys, et cetera).
5: I wouldn't worry too much about your actual car being stolen. I'd consider the alarm as a deterrent to theft of the contents of the car, and also ask if you get an insurance discount (varies from state to state).
6: There is much debate as to the necessity of soldering, and the reliability and/or appropriateness of insulation-displacement connectors like T-taps and Scotchloks. You could read about that here all day.
I want to be careful what I say here, because I have to defend my profession (working for a car dealer expeditor) as well as giving you a truthful answer.
Most new-car installations done for new-car dealers are what we'd call a "basic installation." As in, the unit should be mounted securely, wires routed neatly, connected to the proper wires in the car, no electrical damage to car, no damage to interior panels in the car, and so forth.
Also, the dealer expeditor works as a partner with the car dealership, and counts on future business. If there were ever a problem, you would just return to the Ford dealer, and you can be sure that the dealer would get the installer back there right away with no runaround or hassles.
So, from that side of things, you can expect an installer who's comfortable working on zero-mile brand-new cars, and probably knows exactly how your car comes apart and goes together, since he's likely worked on dozens before.
On the other hand, like I said, expect a basic install. Should be done neatly and cleanly, but expect the alarm brain to be mounted in an easily accessible location (probably driver underdash or at the car's body computer), with just the basic features hooked up (door trigger, trunk trigger, shock sensor, starter kill).
If you want something that will make noise and flashing lights when someone breaks into the car, insurance discount, peace of mind, and something you can roll into your auto loan, the dealer option should be good.
On the other hand, I can tell by all your questions that you might be the kind of customer who's a little fussier than that. If you want the alarm brain mounted in a hard-to-access location, wires disguised to look like factory, extra accessories like tilt, proximity, battery backup, extra sirens, and so forth.....it will cost quite a bit extra in both parts and labor to get it done this way. It couldn't hurt to shop around some of the retail stores in your area.