> How is a "Page" as a separate public identity treated
  > differently compared to a pseudonym?

If I remember correctly there's some anti-spam restrictions on interaction with people not in your circles, but that was a few months ago and they may have been lifted. The current documentation[1][2] doesn't mention any restrictions.

[1] https://support.google.com/youtube/answer/2848323

[2] https://support.google.com/plus/answer/1710600

  > How reliable is it to protect your real identity?

There's no public link between identities, so most pseudonymous use cases are met by Pages. Users with unusual privacy needs will still need to be careful, as adversaries could perform statistical analysis of text style or activity times to determine if two identities are owned by the same person.

But it is impossible to create a page if you already linked your Youtube account with a Google+ profile. I did so because I just did not understood what that page was all about. There is no way back. At least I could not find a way.

Go to https://plus.google.com/u/0/dashboard and click "Create a page"

Yup this works. But this creates a new Google+ page and I have no way to connect this to my (old) Youtube account. I can create a new Youtube Channel and another new page but this had no effect on my old Youtube Account and the data in this account.

Youtube also defaults to my real identity if I visit the viste. If have to manually switch identities.

Edit found the explanation: https://support.google.com/youtube/answer/2657858?hl=en&ref_...