Hi Belen,
I am sure that lots of people saw your very detailed and well-considered question.
I don't know why others didn't reply. I can say why I didn't reply.
Is google plus really useful?
To me, the answer to this is "no". I have no use for Google Plus. I find no value there.
How many times do you encounter a Google Plus page when you are searching for information? For me the answer to that is almost never - and my use of the word "almost" is generous.
How many people do you communicate with through Google Plus? My answer to that is zero.
I believe, and I might be wrong, that Google started Google Plus to offer an alternative to FaceBook - and very few people have adopted it and stayed with it. I know very few people who put any effort into Google Plus.
I started using Google Plus when Google suggested that authors of web content should make a Google Page and authenticate their content by linking it to their Google Plus page. Google did a few things to benefit the people who did this but then any visible effort by Google to continue this effort disappeared. So, I stopped using Google Plus and it is currently completely off of my radar.
That's the input that I can offer. I know that I am not representative of web users in general or web marketing people in specific.
I hope that someone can add information here that convinces me that I can move some of my efforts to Google Plus and find a greater return that what I receive by putting that effort into my websites.