[stumped]

Couple of tests for ya:
Using my Huawei media pad with kindle for Android app. No sd card added.android 9 I think
Put a azw3 book from calibre into downloads folder. Tried open with.. kindle for Android. Got cannot open this file type.
So I looked to see where my Kindle unlimited books were.? . Something like...\Android\ data com.something.kindle\files. Not sure if it's \ or / in android. Can't check while typing
Copied the book to that location. Opened the k for a app. Book appeared and was readable.
So sideloads to there work. You would have to test subfolders. And test mobi if interested. But my existing books from Amazon are kfx in individual folders so I predict subfolders will work
I suspect the app only looks at that one location, which may change if the app itself is installed to a sd card

Ps I am not sure how you can see differences between renderers. I assert that if I matched fonts,margins, etc in all 3 apps, so that all 3 had same number of words, lines etc on a screen, in the same font, and then took screenshots, you could not distinguish kindle from a clone made in bookari or a clone made in moon
Surely the font dictates how each word is rendered, so if 2 apps use the same font then output is identical. I admit to not being a font expert but I am curious. This seems a bit like saying that a lossless wav file sounds different if played in different apps through the same speakers?
What differences would you look at , to determine which app made each wcreenshot