From what I can tell calibre fixes most of the issues and merges the stylesheets. The body text styles were the same with the exception of one book which used the wrong text size (the indented text were smaller than the non-indented in book 7, I doubt that's intentional).
I'll echo the advice of others, use epub->epub conversion or epub->azw3 for Kindles, and select a suitable output profile under page setup to scale down the images. On the other hand, I tried the original and it worked fine, I didn't notice it being unusually slow or anything on my Kobo Clara
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