I notice that your thoughts on the issue of backshifting are exclusively in reference to writing. As far as I have seen, backshifting typically is the norm in writing, and supports your third, fourth, and fifth points. A novel written in past tense, for example, will typically backshift everything, regardless of whether the assertions remain true in the fictional world for the entirety of the novel and whether it is an absolute truth in the real world. But I think this has more to do with there being a complete, inevitable disconnect from the actual present.
That said, I think many writers consider it at least equally valid not to backshift for matters that are absolute truths that also have relevance to the present (so outside of fiction). I would certainly write "I visited London, which is the capital of England" no matter what the context (in past-tense fiction, were "London, the capital of England" suddenly an impossibility, I would probably use "was the capital"). Similarly, as I mentioned before, I would write "Copernicus reported that the earth revolves around the sun in [whatever year]" because this is an absolute truth in the perspective of the actual present, but "Copernicus said that the earth revolved around the sun", because only the context in which Copernicus said this is now relevant. I don't feel that "an unnecessary gloss" is necessarily an inappropriate gloss, and sometimes it is relevant for the speaker/writer to indicate their position.
So in reference to your first and second points, no one needs to stop and think about whether something is true today, nor does one have to consider that a non-backshifted assertion is necessarily true today, because there are specific instances in which, and purposes for which, one would choose to backshift or not.
I simply can't see any lack of clarity in something like "He said he is sick." Of course in writing (barring contemporaneous correspondence) this is nonsensical, because there is no present within the text, but obviously he is currently sick. This wouldn't make sense if he said this 3 weeks ago, but makes perfect sense if he said it an hour ago.
"John's not coming in today. He said he was sick."
"John's not coming in today. He said he is sick."
There is no real ambiguity in either of these, because of the implications within the context. However, if you wish to put implications aside, I put it to you that the first option is the less appropriate: if John was sick, then why isn't he coming in? Sure, we can say "John said he was sick, and he still is", or we could simply say "John's sick", but there's nothing to say that people can't (and don't) say the second of my sentences, nor that they would be misunderstood if they did.