I have looked into this extensively, and have also done a couple stereo swaps on the 5th Gens.
From what I've been able to find, and to the best of my knowledge, I'll offer my answers to your questions:
1.) I felt EXACTLy the same way about my HUD. I didn't wanna lose all the song and caller information that the factory HU provides through the HUD. However, a good aftermarket HU will provide all that information for you on the screen of the HU. All the other features from your HUD that are not associated with the factory HU will still work (i.e. temp, MPH, tach, turn indicators, high beams, compass, etc.). I disconnected the OnStar as soon as the original factory trial was over. I don't need turn-by-turn or any of the other stuff. OnStar also tracks vehicle data like speed and location - so I prefer to not have my car broadcast how fast and where I am all the time.
2.) Yes, you can use the factory location - in fact, that is exactly what I did. I removed the factory mic in the overhead console, and installed the mic from the aftermarket HU I got.
3.) Yes, you can retain the factory USB port location - but not the wiring as far as I know. The reason I've found for this is because the wiring from the USB port splits inside of the loom. There are two power wires and they split off and go towards the drivers side of the dash. The remaining wires split off and go towards the factory HU's wiring harness. What I did that was much easier than chasing down each individual wire was to remove the factory USB port, and install my own USB female port in the same location, run it up to the new aftermarket HU, and plug it in to the back. This way, you don't cut/splice any wires, and retain the USB port location. As far as I know, there aren't any interfaces on the market that retain the full functionality of the factory USB port for use with an aftermarket HU.
4.) All your steering wheel buttons can still be retained, provided your aftermarket HU supports an input for SWC. The Metra Dash kit has the interface needed for this, and my aftermarket HU has the input in the back for it.
5.) You'll need two adapters for the antennas. For AM/FM, you'll need a GM Metric adapter for aftermarket stereos like this... Metra 40-GM10 GM Mini Jack To Motorola Car Antenna Adapter 265-580
For XM sat radio, you'll need an XM tuner like
You will need to remove the square plug end on the factory antenna lead in your car, and then the antenna plug will fit directly into the XM tuner module. You may need to use some tape around it to secure it so it won't work it's way loose.
6.) The backup sensors can work - but this will take some doing on your part. What I did was splice into the factory stereo harness, and run a pair of wires up to the (no longer in use) factory center speaker. I can help you with step-by-step on this if you have the desire to do this, but I will tell you, it ain't easy if you aren't used to doing some wiring. I am, and I'm also totally anal retentive to doing things with insane levels of attention to detail, so it was something that I wanted to tackle and accomplish.
7.) OnStar will still work as normal, as long as you make sure you hook up the mute wire to the aftermarket HU from the Metra Dash Kit (or whatever adapter kit you choose to use).
If you need any help or detailed pictures of what I did to my car, check out these pics... Stereo Install | Facebook
Hope all that helps.