I have experimented around a little this morning in X5. Publish to HTML is intended for web documents. Even in Plain HTML it throws in extra stuff that a browser will ignore but Sigil and an ereader will not.
If you want to stay in WP because it is familiar, then Doitsu's suggestion of copying in WP and pasting into book view works just fine.
You can just try typing into Sigil doing very much what you do in WP. If Sigil's functions seem limited, it is what you will be ultimately limited to anyway. Code view is like reveal codes for WP, though not so easy to decipher. You can find out about HTML at
http://www.w3schools.com/html/default.asp
There will be some items, like footnotes, that will be copied, but linking them to and from the text you will have to do in Sigil. The anchor button puts in a place you can go to, and the chain link a place you can click to get there. You will have to keep track of the names yourself, but since you get to name them, you can make them something easy for you to remember. (Just don't start the name of either with a number..it is a no no.)
If you have images, you will be better off putting them in Sigil. You can add additional files, the + button, and bring them all into your document at once. Then put the cursor where you want, then select edit insert file, which will bring up a box where you can select which file you want to insert.
If it seems a lot of work, it is, if you want to do anything complicated. Otherwise, if it is just something like a novel, just text, you can just type away in WP or Sigil and the primary thing that is a good idea to do is press control enter at the period at the end of each chapter. This will create a separate section for each chapter, which makes it possible to use with older readers and faster for the rest. It also limits the damage you can do in editing while finding and replacing if you confine it to current html file.