Accurately Previewing a Kindle Book Pre-Sale

I'm helping a friend with the ebook version of her novel, which is going to be for sale, among other places, on Amazon, and we're having some trouble with the testing phase. When I send the book to Kindle via email, the process introduces errors that don't make sense and aren't present in the ePub (like, paragraphs that just aren't indented for no reason I can see). Converting it in Calibre and sideloading it introduces different errors that I don't feel like fixing since nobody is going to be sideloading it. In Kindle Previewer, they look fine.

Is Kindle Previewer the accurate choice for figuring out what the book will look like when someone actually buys it?