One can establish a link from a website to a Google+ page by adding a
rel="publisher" link in the head of the
index.html document (for example).
But, should or can this link be set on all pages of the website? Or is it redundant?
If you think the Google+ use of the tag is useful, then check out what Google says about it. To me, it seems that they are saying that you should put the tag only on the most important page of your site, typically the main page (front page).
Yes you can and in my opinion you should. But if you only want to get the site verified and connect it with the Google+ page adding it in the home page is enough.