Your story is not uncommon, unfortunately.
Basic, popular American car that was built in large numbers for seven years, and it takes all day and requires tech support.
8 hours to find and connect 10 wires?
That tells you a lot about the installer's level of experience, and the level of training given to him by his employer.......but I suppose you already know that.
To answer your questions:
---The AUX button
could be connected to the horn, but that probably isn't what you'd want. The minimum time on the AUX output is 1.0 second, and that'd be very loud.
The AUX output would typically be used to open the power trunk, if your car has one.
But, the 160XV (I haven't heard of a 160XVR) has a dedicated horn-honk output, and a Panic feature.
You'd press and hold LOCK to activate Panic. (see your user guide.)
Also, the horn will beep when you lock and unlock using the remote, but that feature can be turned off in programming if you don't like it.
---I don't install Viper, but I think you'd have a hard time buying just a brain, remote, and antenna, but without the cables.....and it wouldn't be much cheaper even if you could.
You could, however, buy an entire new unit, swap the brain/antenna/antenna cable............and you'd end up with a nearly-brand-new 160, and all-new wiring, ready to be installed in another car..........although if you're not able to install it yourself, that doesn't do you a lot of good.
---A lot of installers install the hoodpin switch and control switch; a lot don't. There is a lot of heated debate on the forums about this.
Hoodpins quickly rust and quit working. Customers bump into (or play with) control switches, and then blame the install shop, saying the remote starter is broken.
Regardless, when I worked at Best Buy, it was a terminal offense (you could get fired) for not installing the hood switch.
It's a big liability risk, and they could get sued if someone got injured because the remote starter came on while someone was working on the engine.
I don't remember if there was any policy about the control switch.
CONCLUSION:
I have no idea if store management would take any pity on you for the installer's lack of skill, and your inconvenience and loss of time. But you could try.
If you could find out where their nearest corporate office is, and bring the hoodpin issue to the attention of the proper people, maybe you could get them to hook up the horn feature for free, or maybe even get an upgrade to one of the models that have greater range.
P.S. I'm not knocking the installer, AT ALL. I WAS that guy! It was really good for my career that these big-box stores will hire anyone.........but unfair that they throw the installer into the garage with little training or supervision.