I always use a tab and set the IDE to show two spaces for each tab. Visual Studio has a setting for this. In the same project if someone wants 4, they can just change it to 4 spaces. In the file, its always stored as a tab

I agree. The number of spaces for indentation is completely irrelevant in a style guide as it can be configured in the editor on a personal basis.

No it's not, because it's not just an editor configuration, it's something in the code structure. People with different indentation working on the same files is a problem, so it's really relevant to try to set a standart about this, at least for open source project.

Yes, the code is structured with tab characters, so there is no different indentation. Only different renderings of the same semantic characters.

which lead to very useless and annoying conflict and different coding style in same files

I'm all about two spaces but this is a pointless argument

My first thought as well