The answers to your questions are:
1) Depends. If you do "Upgrade" installation, you can only install to the partition with existing Windows. Otherwise, you're free to choose.
2) Yes. Windows can be installed to both primary and logical partitions.
3) Depends. If your computer is BIOS-based, then no. If your computer is UEFI-based, then yes.
4) AFAIK, no Windows setup has ever erased the ENTIRE hard disk on which it is installed. It should only affect the partition on which Windows is going to be installed. Despite this, you might want to back up your data in case something unarranged does happen.
As alluded to in the last post, you need to use the Win 8 Pro Full Install, OEM edition to allow you to keep all your qualifying OSs. The Upgrade Media, even when using a custom install from DVD, requires that you replace a qualifying OS (Win XP, Win Vista or Win 7) with Win 8. The replaced OS cannot be used until Win 8 is removed.
The qualifying license is deactivated when Win 8 is installed over it. What I did was to create an Image of Win 7 just before I installed Win 8 Pro. This way in the very unlikely chance that I decided to go back, I can just restore the Image. I might have to call MS to reactivate it, but in that case I would need to remove Win 8.
So even if the qualifying license for the copy is deactivated if Win 8 is installed over it, I could still run my original of Win 8 where the qualifying license would be fine. Or are you telling me that Win 8 reports the license number periodically to a Microsoft database somewhere on a server and, if it has been deactivated in the server's database, refuses to run ?