That's an interesting read. I won't even try to pretend I can give a qualified opinion on that. All I can say is that having some foreign object in one's mouth will definitely impede one's speech to varying degrees, based on how invasive that object is. This, however, brings back memories or rather a memory that might be very well related to what we're discussing here. I remember a native English speaker, an English teacher, living in a foreign country as a permanent resident, saying that there was one thing that had always struck him as odd. He, as a language teacher, made some observations about native speakers of the language spoken in the country he lived in. He, with a lot of excitement said this: "We have this common bathroom in the University I work at. A lot of teachers come there to brush their teeth after lunch and they all speak to each other while doing that and they can still understand each other very well even though in no way the can articulate like that." Keep in mind, I can't really recall the exact wording but the meaning. So, yes, I think it's clear that "temporary accent changes" as you put it would be more or less the same between native speakers under somewhat similar circumstances.
As for different food affecting your pronunciation... I highly doubt that. Some fruit could theoratically do that, like хурма, sorry I have no idea what the word for it is in English. Anyway, that fruit can often cause a feeling that all the cells in your mouth have suddenly shrunkYeah, it can be a bit difficult to talk like that, not impossible though.
As for illnesses that affect your nasopharynx in some way... Yes, that will definitely affect your pronunciation, especially when it comes to nasal "problems" as the air flow might be somewhat hindered. Also alchohol, that will probably ruin your speech the most.
Other than that, I think you're trying to read too much into it. All of pronunciation "problems", when speaking a foreign language, come to just this, the language being foreign. When speaking your native language you don't have to think about how you should move your tongue, what emphasis you should put on words, how you should change yor pitch, etc... You just speak, all of that is just embedded deeply in your brain and in your muscle memory. When it comes to your second language, you have to keep track of all that. Essentially your tongue must move differently and you must control it, you have to watch your grammar, your intonation. In other words you're extensively multi-tasking and eventualy it exhausts you both mentally and physically. That's the case with me at least. While I could mantain my "English mode" pronunciation for a few hours, I would inevitably switch it all back to the familiar "Russian mode" bit by bit the longer I talked. It just was way too much load for me to not crumble underYes, it would progressively become easier and easier for me, up to the point where I couldn't draw a line between the two languages any more, but that's a very slow process that requires constant upkeeping. Unfortunately most language learners don't have the luxury of having a lot of people they can often talk to in their targeted language.
It's very interesting that you also mentioned the difference between "native" and "learners" typos. I never really thought of it this way, but I can agree that you can often realize who wrote something by noting where they made their typos or more like analizing their typos. I, for one, often don't type an "s" in plurals, even though I had a proper plural in mind. I don't have the faintest idea why this happens though.
Again, a very interesting read. I just wonder why you really raised these questions in the first place. Is this just something you try to analize and maybe pick up a few useful things from or do you want to completely lose your accent when speaking Russian/Swedish?