Should role="contentinfo" be always added on footer element?

As of now there is no definitive right answer.

If you go by the spec: http://www.w3.org/TR/wai-aria/roles#contentinfo, contentinfo is described as:

"A large perceivable region that contains information about the parent document. Examples of information included in this region of the page are copyrights and links to privacy statements."

So, in this case it should be placed on your <p> tag.

However, pretend you are the user that is utilizing these landmarks. If everyone places the role="contentinfo" on the footer, then that is what the user expects. The user doesn't care what a "spec" says, they just want a consistent experience as they browse from website to website. They are probably expecting footer links and such to be in the contentinfo section, because that's how it is implemented on many sites, and as you mentioned it is the recommended way per many accessibility experts.

I prefer to cater to the user, and therefore I apply it on the footer element, however I keep in the back of my mind that this goes against the spec (or at least my interpretation of it) and that implementation of this might change as browsers and other accessible technologies begin implementing accessibility in a more consistent manner.

I think it should be on the footer tag in your case.

It is meant to give information about the parent document, so I would use it if your footer elements give a good context of the parent.

https://www.w3.org/TR/wai-aria/#contentinfo