I just returned to LEDStrain today after quite a while and noticed all of your replies to my original post. Many thanks for your interest, sharing, and reflections so far. Here is an update about where I am as of now.
Firstly, the Kindles I have been referring to do not have a backlight, this has always been a criterion for me in obtaining a Kindle since I don't trust backlighting (specifically, its potential to cause me problems). The Kindles without a backlight that I have been using have all been Kindle Basics (7th, 4th, and 3rd generations), along with a Kindle DX Graphite.
As to why the newer firmwares have rendered the Kindles unusable to me, I frankly have no idea. It does seem inexplicable, but I'll leave the speculation on this to people who are better versed in technical matters than I am.
So as to where I am now:
I did make significant efforts with Amazon, beyond simply contacting their standard customer service via chat, which predictably led nowhere. I namely wrote several of the top executives of the company and complained in vigorous terms. One of these executives put me in contact with Amazon's "executive customer service," and to sum up a very lengthy and protracted back-and-forth: the gentleman confirmed with the Amazon technical people that downgrading the firmware is in fact impossible, and I was eventually given a gift certificate from Amazon for the amount of a new Kindle Oasis (two-hundred-and-something US dollars). Of course such a new Kindle Oasis would be useless to me, but the two-hundred-and-something dollars can be used for purchasing anything else on Amazon. So while Amazon's automatic updates (a.k.a. "hijacking" one's Kindle) are a terrible practice, and making it impossible to downgrade equally so, I do at least have to give them credit for offering me the gift certificate.
But that is all not directly relevant to the matter at hand, namely solving the problem of now being effectively Kindleless. It turns out that some Kindle models, and firmware versions, can in fact be downgraded by "jailbreaking" the Kindle, which I understand is basically synonymous with hacking it. On the Mobileread online forum, instructions are provided for doing so, and my Kindle 7th generation with its current (updated) firmware is one of those models and firmwares that is capable of being jailbroken. I followed the instructions, over the course of which I ran into numerous hiccups and snags and was obliged to ask for help several times from the forum members. With their assistance I managed to overcome these hurdles, up to the point that I not only successfully "jailbroke" the Kindle, but made it through a number of the subsequent steps. I was truly almost there, I could see a light at the end of the tunnel, and I even had the old, original firmware files ready at hand, which were to be used in the next and almost-final step, the actual downgrade. However, here I unfortunately ran into an irremediable snag -- the PC was unable to "SSH" into the Kindle, which was necessary in order to do the downgrade. The people in the Mobileread forum were unable to help me this time. A few days ago I took my Kindle to a local computer repairperson, thinking that he might succeed in getting through this final step where I had not. However, he ran into exactly the same problem as I had. So downgrading on my own, despite considerable efforts, was not possible in the end.
Parallel with all this, I had been trying to find a used Kindle online that would have an older firmware version that did not cause me problems. I think I mentioned in my previous post that I had ordered a Kindle 4th Generation, also a Basic model, which I assumed, because it is so old, would not cause any issues. However, I discovered that it, too, had been automatically updated and was thus unusable to me. I then reasoned that my only solution would be to find an old used Kindle that had never been updated. I came upon a Kindle Keyboard (the 3rd Generation Basic model, I believe) whose previous owner assured me that he had never updated it. However upon receiving the Kindle, I discovered that it had in fact been updated -- without the owner being aware of it (automatic update, naturally!). Fortunately, I was able to return this one.
I next tried my luck with a used Kindle DX Graphite, which I also found online. The DX is quite old, dating from 2011 I believe, and I was told on the Mobilereads forum that its firmware had not essentially been updated in quite a long time and was no longer being supported/updated by Amazon -- the only recent updates, I was told, were of the security certificates. I thought that I had excellent chances with this one, so I forked out the relatively larger sum for the used DX and discovered, upon receiving it, that lo and behold: it was also unusable to me. So either the DX 1) would, for whatever reason, have caused me problems even in 2011 (or whatever year it received its last substantial firmware update) or 2) the recent updates were of more than just the security certificates. So now I have this beautiful Kindle DX (I'm quite fond of its large size, appearance, and design) that I am unable to use and that I will have to offload in one way or another.
What other options did I have? Kobos and Sony Readers, perhaps. I went to the local electronics stores and tried out the new Kobos on display -- however, all were no good for me. Sony Readers have been discontinued, and consequently are all at least a few years old, but they have been acquired in some capacity by the Kobo company, which has loaded onto them some basic sorts of updates in the meantime. I found someone selling a Sony Reader in my town, visited them at their house, and tried it out -- alas, no luck with this one either.
I have not yet tested the ONYX Boox e-reader (nor have I tried the ONYX Boox Mira or Dasung Paperlike external monitors -- they are quite expensive and I would need to order them from a vendor with a return policy).
I'm planning on trying something else now, I'll report back.
For the sake of completeness, I should mention that I have not tried fiddling around with the fonts of my Kindle. I don't expect that this would make a difference, as I have never noticed varying sensitivity with varying fonts before (on my PC, for example), and I don't understand how it could help (different letter shapes?), but I suppose that for the sake of exhaustiveness I ought to give it a try.
I hope my experiences can be helpful to you all in some way, and it would be great to hear your ideas and experiences. What a long ride this all has been -- after so many years of happily using my Kindle with no issues. The moral of the story I suppose: never leave airplane mode!!