Rare flat adverbs and fixed phrases

For some words listed as "flat adverbs" in the various sources on the internet, I could find only very limited evidence that they are actually being used as flat adverbs. In other cases, I stumbled upon specific phrases in which words appear to be used as flat adverbs that will usually just form the regular adverb or not be used as an adverb or submodifier at all.
Those special cases are listed in this section.

current, even, flat, full, soft

current [rare/split] - "current", derived from Latin currere = to run, means approximately "happening now", and the adverb "currently" is explained as "at the present time" by Cambridge Dictionary.
In the example "to be paid current", however, the word has a somewhat different meaning that can be interpreted as closer to the original meaning of the word - the payment happens parallel to the work rather than quite some time later, thus "current" here means "ongoing, continuous" and can be compared to the idea of the "running commentary" that comments a development parallel to the development taking place.
It appears that the flat adverb is being used here since "currently" has already got a different meaning (as in "right now" or "very recently").
mentioned on the LEO forum

even [rare?/split?] - "even" is listed as a flat adverb, but all the examples I could find used the regular form (e.g. dividing or distributing something evenly), so that seems to be the safe choice. There is, of course, also a way of using even as an adverb, but then the meaning is completely unrelated, so I am not sure if that is what the "visual thesaurus" meant.
mentioned in "When adverbs fall flat"

flat [rare/split?] - The blog Dailywritingtips lists "flat" as a flat adverb and says: "The flat and normal senses of this term are closely related but distinct: 'I was turned down flat', but 'I was flatly refused.'"
However, the idea of distinct meanings of the flat and regular form is not quite clear to me, and was also refused by a British native speaker (#54). Searching for possible appearances of "flat" as a flat adverb, I found the following terms and examples:
The already mentioned "to turn somebody down flat", "flat out", e.g. in "He's been working flat-out for the last two months", and a bunch of compositions such as flat-footed, flat-chested, a flat-bottomed boat, a Flat-Coated Retriever. I am quite sure, however, that in these compositions, "flat" is indeed an adjective: In the case of the Retriever, e.g., it doesn't make much sense to say that "the Retriever has been coated flatly".
That leaves "to turn somebody down flat" and "flat-out" as the two examples I could find in which "flat" serves as a flat adverb, and thus I would call it a "rare" flat adverb.
taken from "Flat adverbs are flat-out useful"

full [rare] - "You know full well what I mean" (#62) -- "Paid in full" (e.g. a movie from 2002)
While there is no source listing "full" as a possible flat adverb, there are at least those two fixed phrases in which "full" technically serves as a flat adverb and "fully" would be simply wrong.
Usually, however, the regular adverb is being used, e.g. in "fully loaded" or "I am not yet fully convinced".

soft [rare?] - "soft" is listed as possible flat adverb, but seems to be mostly used in the regular form. The various examples I could find all used the regular form, e.g. "talk softly" (as in "talk in a low voice"), "the wind blew softly" (as in "gently"), the room was softly lit" (as in "dimly lit").
On the LEO forum, "soft" as an adverb is not supported, apart from the regional usage "don't talk soft" as in "don't say such silly things" mentioned by a British native speaker (#55).
mentioned in "Flat adverbs are exceeding fine"