Programatically adding new CKeditor instance What is the trick to ckeditor in D8? It's woefully under documented and CKEDITOR / CKEDITOR.instances seemingly return nothing, despite showing multiple instances of ckeditor instances available. CKEDITOR.replace() seems to give a config error (missing 404 js files) when you try to target an element. 

I can target and get data back for CKEDITOR.dom.element.get('edit-comment-body-0-value') but once try to use getEditor(), it automatically returns NULL.
You're not supposed to use the CKEditor API directly. You could do that if you want to use CKEditor on something you control. But your DOM ID (edit-comment-body-0) indicates you're trying to do this for a "Formatted text" field in Drupal 8, which uses the filter system.
If you'd want to attach CKEditor manually, use the JS API provided by core/modules/editor/js/editor.js.

The integration between Drupal 8 and CKEditor is very solid/deep, in that Drupal 8 automatically configures CKEditor to match the text format's filter settings. And that integration is not tightly coupled to CKEditor: there's the Text Editor (editor) module that allows any text editor to integrate in a similarly deep way. But it means you should always use the APIs that editor provides.

For API docs, see:


https://www.drupal.org/developing/api/8/editor
https://www.drupal.org/developing/api/8/ckeditor
https://www.drupal.org/developing/api/8/filter


For the concrete relevant code, see TextFormat, editor_element_info_alter() and \Drupal\Editor\Element::preRenderTextFormat.