Sorry to hear about your bad experience. Stuff like that really bugs me too! There are connectors available to allow for installation of aftermarket radios by plugging onto the factory connector. Using these allows people to revert back later if they wish. Sounds like they either don't know their business very well (didn't realize they exist), or they didn't really care. It also may have been the last owner who told them to just go ahead and cut it and not the fault of the shop. The connectors I'm referring to aren't stocked at very many places and it likely would have meant waiting before the stereo could be installed. Some customers are impatient and wouldn't be willing to wait.
There are options (as you've discovered) to add the functionality of steering wheel controls to aftermarket radios. There are lots of users here who have done this. My experience is limited to stock equipment, but I'm sure someone will step up if you still need some assistance.
By the way, the part in your original post is correct. It'* called a Dock-N-Lock connector. It was only used in a few GM vehicles including certain models years of Bonneville, LeSabre, Park Avenue and Aurora (I think that'* all of them).
Hope you get everything working to your satisfaction.
StingRay Bike w/cards in the spokes, Weeeeeee!