This is an interesting question but may not be worth the time to track down. I did a google search on the term after reading this question and found contradictory uses of the term. On the one hand several academic sites refer to case (1) as the "original work" not an arrangement. And they also refer to an arrangement as a variant of the original work by application of new or alternate harmonization, changes in tempo, style, rhythm, etc. So it would seem that we are discussing "original work" (which by the way has an arrangement) rather than some sort of arrangement of the work after the fact. On the other hand I find several examples of statements like "after much efforts we finally tracked down the original arrangement of Misty..." or some other tune. I think in this case they are referring to the arrangement developed by the composer or the first recording of the song. Sometimes composers do not do their own arranging. They may work out a tune on the piano and hand it off to a partner to arrange for orchestra or band.
Based on my search I can add a third option to the mix, just to make it clearer? Some may refer to an original arrangement of a piece by a particular artist or genre. As an example (potentially a funny one to some) consider the orchestral arrangements of Metallica's work. This is not the "original work" and it is a "first occurrence of such an arrangement", e.g. original. Decades after the fact several variants of this arrangement may have been written and produced based on the creative decisions of different conductors etc. Now "original arrangement" does not refer to Metallica's recorded albums. I might refer to this as the "original arrangement for orchestra" as someone may produce an arrangement for big band, or a Latin percussion group.
And there may be a fourth case. That of the original composer creating a NEW arrangement of their own tunes. Miles did it. Many people do it. So then what is this? Still a new, original arrangement of an original piece by the composer (not the original composer as that would be redundant, unless it's Stairway to Heaven (possibly)).
When someone makes their own arrangement of a piece I think it makes sense to call that an "original arrangement". So if I saw that in a description I would think that I am about to hear a version of something created by the people who posted the video. You cannot stop people form using terms out of common context and quite frankly "original" can apply to both cases.
In my opinion original work should be used for composer's first arrangement and original arrangement to refer to new arrangements of the piece by other artists. It may be useful to use other adjectives such as first arrangement, composer's arrangement, etc.