I have diagrams pulled for an 07 and a 2010 but it shows many of the wires to be the same:
I haven't done this personally but I know other people have successfully mounted the newer stereo in. I'm not sure if the pins are the same but the plugs are just different or what. But hopefully these diagrams can get you a bit closer. They include all possible options.
As for the antenna adapter you can definitely purchase one. Most are to connect an OEM antenna to an aftermarket stereo, but I'm sure it will also work with the newer gm stereo.
Could always do a junkyard run and pull some wires...
Yea all those pins/wires sold pretty quick. There's cluster connectors with wires attached for sale on ebay. Could always get one and pull the pin out.
I may be going to the yard soon so maybe I'll grab a few, I'll post if I do.
I wired up my stereo using the PAC OS-311B Harness, It retained the RAP and onstar, it also brought back the chimes but I left the speaker unplugged because it's nicer without IMO.
I don't totally remember my install. I THINK the dic connector just sits there tucked behind the stereo. But I don't recall if my PAC harness had a connector for it to retain some of the functions. I'd have to pop it out and take a look I guess.