Unopened is unopened.
NIB is new unused, complete, free of un-noted, non-production flaws.
If handled, um.. without, you cant know anything without inspection, so..if it possibly lightly handled/tested(though tests type/time/distance should be noted), even displayed for a short time, still NIB to me..if complete.
"Like new" lightly handled, minor flaw from?, excessive testing.
Proper tests are done on rollers now days, which don't leave scuffed flanges on pilot & trailing wheels.
A "display model" notes the possibility of heavy handling, and/or some customer operations during display, therefor "used". But the possibility of still being "like new" exists.
"Open box"- Not tested, or poured over visually, left to the handling of the general public, or pulled from a back room without any idea of its original reason for being there, returned.
The dealer did fail you with the completeness of the packaging. If this were an "investment" you would be at quite the disadvantage.
My bet is it was grabbed from a test bench for shipping to you, or put away too quickly by a handler. Might have been the "new guy", lack of, or too much coffee that day, etc. If that's how it was... on a shelf.. even in a case, its "Open Box"
Two mistakes in twenty five years sounds pretty human to me. Give them a chance to make it right. Delay only makes it harder on you both, and with the packaging.. well how long do you hold on to loose packaging?
While base expectations may not have been met here. If you expect 'mint" as a buyer, you should both ask about it, and should state it pre-purchase. And do so repeatedly, every time. Maybe shop in person if you "tend to be fussy".(not an accusation, I can't know)
Maybe dealers should consider a premium charge for pre-sale inspection, and grading of "mint" C-9, C-10 items. It works well enough in other "areas" of collecting.