2.: Yes, if you run multiple XP VMs, you need multiple XP licenses. Win7Pro includes the XP Mode license, so that's one.
It looks like there's some leftover stock on the Amazon Marketplace, but it may take some digging...
3.: As mentioned above Win7Pro includes a license for "XP Mode" - that is, one copy of XP, run in a VM hosted on the PC.
5.: Yes, using vCenter Standalone Converter.
Of course cloning has its own issues, both technical (mostly conflicts with system-specific drivers) and legal (OEM licenses are not transferable, and some OEMs may prohibit transferring the installation, too. And the installed software can have similar restrictions, of course).
Regarding Apple: No, that would violate the Mac OS license.