You can disable Editable on the site level in /user/config/plugins/editable/editable.yamlwith setting editable_self: false.
By using shortcodes in the "primary markdown file in the parent folder of the modular page" you can control what or which parts of a modular page will be editable in the frontend. Each shortcode gets it's content from, let's name it, a 'source page' that you specify in the shortcode.
These source pages should (thus) be normal pages where the page content is delivered directly by Twig. This means the template used for that source page should simply insert the page content with {{ content }}.
Currently Editable only supports shortcodes when the editor in use is ContentTools. In theory shortcodes could also work with the markdown editor SimpleMDE. The reason I choose not to support it is the way the user interface will look. Having multiple editable regions on a single page is one of the design principl es of ContentTools and this (thus) works very well. All regions share the same tool button panel.
With SimpleMDE, each region will get it's own toolbar at the top of that region making a very fuzzy interface.
It's not my intention, and beyond my capabilities, to create the definitive and perfect frontend editor plus user interface for Grav which will be well designed, easy to use and supports the various use cases we discussed here. I wish I could :)
My aim was and is to bring frontend editing to Grav or rather give developers a head start or starting point to bring it further.