I have found that a well-formatted epub from Sigil works very well on Nook and Kindle (and Kindle apps including IOS). I don't own an iPad and have never bought the iBooks version of one of my books.
Also, because of my distaste for the closed Apple universe, neither can I upload directly to Apple. I previously used Lulu for this but more recently have switched to Draft2Digital. I find that my Sigil epubs from D2D look better on my Nook device than those I uploaded directly to Nook Press, as it is called. (By "better" I mean they look as I intend them to look, and as they do look on Kindle readers.)
I upload the identical epub to the KDP and to D2D, except for an included cover in the D2D version.
You can see my style sheet at
http://notjohnkdp.blogspot.com/
Oh, and I should add that I don't mess with line spacing! I use top-margin on chapter headings etc and on certain paragraph styles. The Kindle reader last year increased its line spacing to the point where it's almost 1.5. Nook is not quite so spacious, but there is certainly lots of room there, something like 12 on 15 point in a print edition.