Currently Being ModeratedFeb 16, 2013 11:59 AM (in response to Zippy Two Shoes)
The advatange to installing parallels through boot camp instead of on its own is that then you can also use the same PC setup in "native" speeds. Boot Camp will run Windows faster than through Parallels in MacOS (though often the difference isn't consequential), and there are probably a handful of software programs that will also run more smoothly in Boot Camp than in Parallels.
I don't recall whether it's possible to uninstall boot camp and re-use that Windows install as only a Parallels installation. But as a worst case, you'd just have to copy any files you store in Windows to your Mac, eliminate the boot camp partition, and create a new Parallels installation with any Windows apps you had installed. Not something that can be done instantly, but not terribly time-consuming either, unless you have a bunch of Windows apps.
To the third question, in my version of Parallels (7.X), you choose Preferences under the Parallels menu in the MacOS menu bar, click on the USB tab, and then there you can choose whether USB devices like thumb drives show up on your Mac side, or your PC side when Parallels is running, or ask you each time you add a new device. But it has to be one or the other - Mac or PC - you can't have it both ways when Parallels is running.
Hope that helps...