I cannot find any policy on the matter through their websites.
Nature Review articles do not need to be invited. And review articles are often not "original research". Nature Reviews http://www.nature.com/reviews/index.html accepts these. So the answers to your questions are "yes" and "yes"
In addition to @mankoff's answer, be aware that it is possible, and indeed common in some fields, to email an editor with an idea, or draft, of a review paper and ask if they'd like to "invite".