Another mazda6 user! Woo!
Check out the link in my signature. Also.. http://www.clubatenza.com < mazda6 forum
http://www.clubatenza.com/forum/viewtopic.php?t=114 < my carputter thread!
To answer your questions:
1) If you're thinking of doing a map pocket install, it's possible but I found that it's too far for my reach inside the pocket, which kinda ruins the touchscreen functionality. Hence the reason why I installed it outside the pocket. Otherwise, you could modify/remove the HU HVAC controls and put in the screen in there.. But that requires a bit more work with bondo/fiberglass and relocating the stock HU hardware.
2) No. People have tried but to no avail. If you remove the HU you will loose the top LCD module with it.
3) Easy, check out griffin, you could control it through that. You might also want to check out the audio / electronics section on clubatenza for the aux-in sticky.. it has some info about overloads module. It will allow you to keep the steering wheel controls and interface them through RS232 to your carputter! Neat huh!
GL on your project. Make sure you post some pics once you get it done!