Is manga censorship in Western countries always done with author consent? Censorship is usually seen as an act against the author's will. Talking with an editor in my country (Italy) working for a publishing house in the erotic manga market, he tells me that censorship for erotic manga is done in two verses: removing censor bars and obfuscation necessary in Japan (decensorship) and removing elements not suited to Western audience or in a gray legal area, e.g. child nudity. Both of those censorship and decensorship are done with the author consent, becoming a self-censorship and generating a new derivative work featuring important changes.

Shortly: is censorship of manga always done with author consent both in erotic and mainstream publications during localization? Is there a common practice around the world on this topic about manga localization? Are there any cases of censorship of manga publications done without author consent in Western countries?
You are over-thinking it. I guess we only talk about official localizations here, since in non-official ones, anything can be done.

When an official localization is made, some kind of contract is being signed between the possessor of rights for the manga (the author, a publishing house, doesn't matter), and the company that is going to make the localization. 

Since this contract is an legal document, it covers, among other things, the limitations (if any) that will be set for the localizer. Obviously, since different countries have different laws regarding nudity in press (as in your example), the contract should probably have clauses that regulate such possible changes (censorship or decensorship). 

Whether or not the author himself is aware of this, depends. Sometimes the holder of the rights will inform him, sometimes it will not. For example, when official localization of Spice and Wolf into Russian was being made, the author himself didn't know about it, since it was the publishing house who was holding the rights on the manga, hence it was regulating all the localization aspects. 

And after all, if the laws of some country forbid, for example, nudity in press, then there is no choice. You either change the manga to meet the laws, or it is not going to be published.