Section of paper, excluding sub-sections, without using section numbers

This often serves more as an abstract than an introduction, but I don't suggest calling it that.

Your case of referring to (presumably) someone else's work probably is served best by "section introduction". Your second case ("please review... ") could use the same, or it could use "3.0" despite the lack of an explicit ".0" in the document. You'd be widely understood, though of course some people would miss your meaning.

If you're using version control for LaTeX or something like that, and you're avoiding referring to section numbers in your comments, you should refer to the section title in some way, except that too can change - something like "the intro to the methods section" is probably your best bet there.

If you're in full control of the document, I suggest minimising the need to refer to such parts of sections. You can usually refer to the subsection that discusses the material fully, or to the entire section. I'm not trying to dissuade you from introducing your sections this way; it can work very well, and even better at the chapter level.