Hi,
I think the answer why utility widgets really are Level Editor Utility Widgets is because the request comes from LevelDesign/LevelArt/TechArt and that no one thought they could use them for more fun stuff. Or at least that the prio to have them work in other part of the editor was/is low.
As it was very easy to get utility widgets working in the Blueprint editor like I do in the ElgKismetEditorWidget plugin you linked to 
And yeah you can mostly add/edit/remove stuff like variable, functions or macros but it’s mostly because I didn’t have any specific user case plus it would be somewhat cumbersome to create and link nodes with blueprint nodes.
One way you could “generate graphs based on some input” is to use the copy/paste feature and manipulate the clipboard text data and insert/edit/remove nodes based on some input.
But yeah the copy/paste feature is also kinda halfa**… 