Wow, I read through all the replies here, and my first reaction was that Amazon (and Kindle) seems so incredibly daunting that all I wanted to do was give up on even bothering to try, and just crawl back into bed and lie there in a fetal position!
But then I mustered up my courage (and curiosity), got a second wind, went and downloaded/installed KindleGen, Kindle Previewer and Kindle for PC, and then opened up my ebook -- i.e. that first one I did (with that "olde" second-half), not the one I've been referring to here re this background image question -- and checked it out to see just how badly it would fall apart.
Well! Despite my apprehension that nothing would work right, on the contrary I'm amazed that -- for the most part -- everything seems to look just fine!
To summarize...
- In all three Kindle Fire tabs, everything seems just "perfect"! I haven't gone over it all with a fine-tooth comb, but as far as I can see everything is just fine, no problems that I can "see".
- In Kindle Paperwhite, things seem surprisingly just fine, too. Obviously the embedded fonts don't work, but even still that "olde" part seems to work out okay. The only real issue that I see is where I'd used two embedded ornaments, which now show up as letters instead, but I think that could be resolved by going into Font Forge and putting those ornaments that I wanted to use into different slots. For example, I'd used a nice, fancy ornament underneath my chapter headings, and right now that ends up coming out as the letter "d" -- but I could put that in the slot for the em-dash (or something) instead and then it wouldn't look so bad. Well, better, at least.
- in KindleDX, none of my full-page images show up -- but I suppose those people simply won't know what they're missing (and I can only presume are used to stuff being "missing" anyway). The only real issue that I see with other images are the images I'd used as dropcaps, which don't resize down to a reasonable size and are HUGE (comparatively)... but once again, I would assume that people on that platform are used to things often looking "funny" in that sort of way?
I realize that it would be nice to have things degrade gracefully for certain platforms (or older devices), but to what extent do we sacrifice all the nice things we can do with more modern devices just to accommodate those older devices -- especially considering that people using those must surely be used to seeing lots of "crap"? And even the issues that I do have with that book of mine don't seem THAT drastic (and the issue with that embedded ornament font I could surely resolve in the way that I mentioned).
So I don't know -- as usual.

With that first book of mine, that people kept saying over and over and over again that it "Will never work in Kindle", at least as far as viewing it in Kindle Previewer goes it seems to work out either
perfectly (as far as I can see) or at least reasonably, forgivably okay (oh, and also in Kindle for PC it's virtually "perfect").
So now what? I haven't tried out my second book yet -- where I was using that background image for each/every chapter, and I presume from what people have been saying that aspect of it will just fall apart (in some Kindles, anyway) -- but as far as my first book goes, with it having seemed to come out just fine, really (with a couple of small, easy fixes in Font Forge), am I missing something?
I know this is off-topic from the original thread here -- let alone the book that I was talking about for that issue -- but if my first book seems to work fine, what's to prevent me from just going ahead and getting an account with Amazon and loading 'er up there (i.e. this mobi file that KindleGen made)?