Newbie trying to make a section menu. Best menu functions?

The Context: I'm still inexperienced with WP and PHP. I'm trying to make a section menu plugin, to fill a common need that's not being entirely met by any existing plugins.

Simple Section Navigation gets the closest - allowing a menu on a page that starts at the top level parent (excludes top level siblings), includes siblings of the current page (in the same section), and also shows children of the current page. An implementation example of this is here:

The Problem: Simple Section Navigation uses the Page list/hierarchy for building its menu. I need a plugin that builds it menu from WP's menu system.

The Question: What WP function would it be best for me to start looking at for giving me menu information for this? I'm expecting to be able to give it the ID of the menu I'm interested in, and be able to walk through the array of data to find my current menu item based off my current slug, get its parents, children, etc.

Thanks for any quick start info you can give me on this :slight_smile: