soon after my last post to you, it occurred to me that if indeed this works as you say with export (and you were very thorough so i don't doubt it, but trust and verify seems to be the path i should follow), that perhaps i know why you are getting these results and how you can influence the outcome and have the exported book open where you want it. that said, my work flow is via preview (and not export), which i like better.
just a few weeks ago i was wondering why people use export and not preview when the target is ibooks. i came to the conclusion that preview created its copy into iTunes/ibook but export allowed you to place your actual ibook in ibook format anywhere you want. since preview to iPad stopped working for me and others (documented on this forum) under iOS 8.something, i gravitated to preview to mac, sync to iPad via iTunes, which i was clued into by the great tom g. whether this and any other error was fixed by iOS 9 i do not know as i am too busy at this point trying to finish my book. but soon.
i hate to speculate what might fix this, but right now i am pressed for time to finish my book, but i will because you might have the time to see if it works. it shouldn't take long for anyone to test, but i have to stay focused on my current project. it is easier for me to tell you and for you to test it out, especially if this is critical to you.
this is either (1) a bug in export which cannot be fixed or (2) requires what i am about to tell you or (3) someone else might know.
this is #2 above, which i would try after i verified your findings. open up your book in ib author. make a slight change to the page where you want the book to open up. easy to do on one of the special pages, like the copyright page, perhaps harder to do on the intro media. just delete a word or a letter and then retype it to fool ib author into thinking a change was made. for the intro page, you are likely to have to drop you intro movie into the widget again to make a change. i think both of these would fool iba that an actual change was made to that page, unless he has some special high level checksum intelligence which requires a real change twice, once to change and one to change back. my wild theory is that export opens to the last changed page. i have to think there is an autosave done before the export, but to be on the safe side, save explicitly after you make the change.
every thing that you tried did not make a change to the actual book, so whatever is telling it where to open is not changing. of course, preview does not work this way, but export is no preview.
i have no idea whether this theory will bear fruit. if not and no one steps forward with the definitive answer (step 3 above), then it is time to call apple support if this is critical you you. if you bother to try this, i hope it works for you. i think it is worth the try if you are the experimental type.
best of luck again on your book.