1) as a colorist, you don’t edit footages, but you get them from editors. Editors will NOT use Davinci, sir that’s why you’ll have to import their cuts, and match them to the offline edit reference file. Of course you can edit your footage in Davinci too if you’re also the editor of the project.
XML import is a way to act like a bridge between editing softwares and Davinci, just as AAF, and EDL.
2) usually yes. You can also colour correct source footage, but that’s not an usual way.
3) Lower right corner there is a slider, or Alt/Option and scroll
4) IMHO if there’s and issue with the XML import, then it has to do something with the settings, and not the applications. IF your FCP project is linked to the original source files (which are name, reel name, and timecode correct), your XML points to these files, then Resolve should find them in the given folder in no time.
5) You can colour grade the raw files, but you will have to face with two things: more things to render, and after edit you don’t know if your grades will be consistent between cuts or not.