How to include nested addon/package sass in addon/styles/addon.scss for use in engine addon.scss

This is a complicated question that I've spent over a day and a half trying to figure out, and I have very little understanding of the way ember cli and broccoli work in the index.js file of an addon.

I have a project that includes a "core" in-repo-addon, and some in-repo-engines that consume their sibling core addon.
In the core addon, I'm using a nested addon (ember-sass-bootstrap), and am trying to provide access to the nested addon's sass variables and mixins to the engines in the engines' addon/styles/addon.scss file instead of app/styles/app.scss, so that I can lazy load the engines' styles only when required. Putting the styles in app/styles/app.scss would require them to be imported into the parent app's app.scss, which I'm hoping to avoid.

I've learnt how to be able to import the core addon's addon/styles/addon.scss in my engines' addon/styles/addon.scss files and got that part working, but I can't figure out how to be able to import the sass files from core's nested addon into core's addon/styles/addon.scss file.

In core's index.js file, I've tried things like sassOptions-includePaths, treeForStyles, treeForParentAddonStyles, mergeTrees inside the "included" hook, etc.
I don't understand much of it, and am hoping someone can help me out.