Is it possible to protect or encrypt complete opera user file including bookmarks, history, settings etc. so, that a different profile can be used whenever needed? I wonder if there's a built in or third party plug-in would allow to do that. I would also like to know the manual way if there's one for Opera 11.
I don't see how encryption or write(?) protection could help to use multiple profiles in Opera, but since this is what you probably want: Yes, it is possible.
For the current version on Winows this requires renaming of a existing profile directory, some INI editing (fixing paths) in your opera6.ini and running
opera.exe /settings "path\to\your\opera6.ini", as described in detail on http://operawiki.info/operaprofiles.
For the upcoming Opera 12 it is easier: just run
opera.exe -pd "path\to\opera\profile" - Reference is here: http://my.opera.com/ruario/blog/2011/09/28/the-pd-personal-directory-switch
No, you have to encrypt your different profiles manually. For which if you want to do in the same Windows account you will need something like TrueCrypt.