I think you have found the answer: Sideload as an epub.
I doubt if the Android or iOS app will have any special handling for sideloaded kepubs. I think it will just treat them the same as any sideloaded epub. But, you could try the double extension of ".kepub.epub". This is what is used on the devices to trigger a book being treated as a kepub rather than an epub. But, a very quick test here doesn't show any difference with either file name.
And looking at your post in the calibre forum, I assume the differences you are referring to are something you see in on your Clara HD between the kepub and epub. If so, they are probably differences between how the two renderers work. But, you would need to load exactly the same version of the file to both renders to be sure.
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