I FOUND THE SOLUTION!!!
To the issue re full page images adding an extra blank page when viewed in portrait mode...
For whatever reason, what was causing the problem was that even though I had each/every full-page image done up as a separate HTML file, in those I had been linking to my style sheet as well -- by simply deleting that, it got rid of the problem! I don't know what it is that I have in my style sheet that causes that extra blank page (only in portrait mode, in iBooks), but that's what it was. In fact, I've now set my image sizes to 100% (not 99% or 95%) and they're coming out fine, both landscape and portrait.
So that problem is solved!
I'm really starting to wonder now, though, if perhaps I should just design specifically for iBooks. I realize that that will limit my audience (and potential income), but I'm not really all that "ambitious" anyway -- I'm semi-retired now, I don't have a great enough income to go out and buy any/every ereader on the market (past/present/future), not to mention the frustration level of trying to design things that work in "every" platform.
I've got an iPad, and while I'm sure opinions might vary on this, from all the ereaders that I've seen, and all the different apps that I've tried, none seem to work/display as nicely as iBooks does -- I just
love how the latter lets you actually display the book so that it looks like an actual "book" (with the simulated gutter in the middle and stuff)!
Anyone have any thoughts on that? I don't mean on my opinion of iBooks, really, but rather on my limiting any ebooks I might create specifically to iBooks? Like I said, I'm not really "in it for the money". It just seems so impractical for me, in my position -- both financially (I can't afford to buy a ton of different readers to test things) and "exasperatingly" (as far as designing multiple versions of each book for all the various platforms).
If anyone thinks I'm making a big mistake in thinking about limiting myself in this way, please don't hesitate to try to nudge me -- with valid reasons -- in a different direction. But at the moment, this morning, this seems to be the direction that I'm leaning to.
I'd love to hear others' thoughts on that -- and in the meantime, thank you SO very, very much to everyone who has responded to me with all their help on this little first-time project mine so far!