As far as page breaks go, I'm learning "on the job" about this. Trying to understand why Calibre split the manuscript (docx) the way it did, and how exactly the splits are supposed to be used. I have noticed that end of each split file there is something like this:
<p class="block_14" id="calibre_pb_5">*</p>
</body></html>
I've been trying to figure out what the "calibre_pb_5" id is used for, and it's odd to me because it is referenced nowhere I can find. I assume the "pb" means page break, so now I'm guessing (I do a lot of guessing if I can't find an answer) that these references trigger page breaks (seems so), but what else they do I would love to know.
So I may join some of the files where Calibre thought a page break should to, but I don't want one. And then split files where I do want a page break. But I'm doing without really understanding this bit of code at the end of each split.
Oh well. Not your job to give me a Calibre course! But I am doing better now that I jettisoned the Kindle Previewer, which seems to be worse than worthless. Instead, I periodically send my epub to my Kindle library. Then I can view it on my Pixel phone, my Kindle Signature Edition, and my Samsung tablet. As soon as it looks good on all three, I'm done. I have a proof of the paperback, and it looks fine and is ready to publish, but I'm still hoping to have a Kindle version simultaneously. And my most recently revised self-imposed deadline is Tuesday, so it'll be within this month. Somewhat arbitrary.