The ability for pandoc to create EPUB3 is neither here nor there.
epubtest is a pipe dream. I will be impressed when I see just one eink device anywhere that is EPUB3 compliant. It is an unwise move to cut yourself off from all the devices that don't support whichever insignificant shiny-ness of the day is being enthused about.
So yes, if you are designing fixed-format/multimedia books, then by all means target iBooks as a virtual exclusivity.
calibre and Sigil are not iBooks creation tools, they are ebooks creation tools. There must be an actual benefit to putting in the work to handle EPUB3.
And for all your excitement about halfway support (really?
) the vast majority of books aren't using a single feature that necessitates EPUB3, which is probably why they keep making EPUB2, which is probably why few have bothered to make apps (and none have made e-ink ereaders) that handle them.