This will be quite a bit so Ill have to chop things up.
First we start with Kiki, the one that started things for me. Originally purchasing an Ebay listing from Japan of a supposed 30 year old stored poster which I was always skeptical about given its great condition. Here it is side by side with a variant (teaser?) that I got in this week:

To the left is my first one of questionable original and right beside is what I assume is the teaser variant with minimal text on it. Sorry for the quality, it was quite hard capture these without blowing out the brighter parts, I had to play with the ISO a bit on my phone camera. Regardless its mostly just to compare colors.
I figured it may be interesting since the teaser variants have never been officially reprinted to my knowledge and probably arnt the most attractive to forge either. But I took note of only minor differences in terms of colors like Kikis dress being slightly bluer and skin tanned on the teaser compared to the others slightly more purple hued cloth and whiter skin:

Looking at the backs might reveal why because in terms of condition the back of the teaser is indeed starting to brown a bit compared to my other which is perhaps not minty white but close to it. A smell test also yields a more positive result for the teaser which is much more aged in smell. Even though Im a relative novice to posters Im quite familiar with vinyl record collecting and have lots of experience with the vintage smell of paper and cardboard all the way back to the 50s through that medium.

In general the paper feels quite similar however, in thickness and texture, both being somewhat glossy to the touch on both back and front. This is sort of a conundrum because besides the smell and backs browning / foxing Id expected the teaser to be thinner in feel which will become more evident with the other purchases in this bunch I got, considering this is one of the older Ghibli movies.
With closer inspection the plot thickens:

I thought the smoking gun to crack this particular case would be the Eirin mark which I never found very clear, although to be fair, this is one of the smaller ones Ive seen for old Ghibli film posters. To the left is again the first poster of mine behind acrylic and to the right the teaser under some thin polypropylene protective film. As soon as I saw the first posters blocky kanji inside the Eirin circle under my USB microscope I figured the reprint method just couldnt depict the symbols accurately enough and just filled in the holes making them almost completely solid blocks of black. But looking at the teaser variant I dont see much of any difference in this area, only the M at the bottom looks to have a cut at the top. The main oddity I found was the slightly moved placement of the circled C. Overall the teaser poster has about 3mm extra height at 729mm compared to our other candidate at 726mm which cuts off the image at the bottom a bit sooner. Perhaps this is why, or indicative of something else as well? My other posters also seem to vary a mm or two in size at times, is this within margin of error?

Sadly no bullseye this time. My assumption is still that my first poster is a reprint, but perhaps from the Lawson 2001 run which would still make the 30 year storage claim somewhat fit, since these were packaged in protective plastic and cardboard and seem to look quite minty still when sold online.
Or is it possible the teaser variant could also have had reprints for it? It might be that these variants were done later and for a purpose other than theatrical use even though they have a "not for sale" statement beside the Eirin.
Theres also the white Dolby variant of this poster which seem like a safer bet than the all black credit ones as these were also never reprinted officially to my knowledge. The only thing Ive heard about these is that one seller claimed to recall getting one as a bonus for purchasing the soundtrack at the time, if that still counts as an original poster Im have no qualification to answer but its unverified as of now as I know of another poster variants which indeed came with the soundtrack so perhaps it just a mistake in memory. But it might explain the lack of numbers in the Eirin mark for some of these.
Finally I also got this Kiki poster:

Which is another good reference point since these are unlikely to be faked Id imagine. Again the age test qualifies with the smell and back, although mine was not the best stored copy. But the paper thickness still illudes me. This could again be a poster made years after the movies release, more so this one than the supposed teaser variant anyway. Other Japanese poster experts will have to chime in to the papers commonly used during these times to help verify if this eliminates the possibility of its age authenticity or not. But thats all I have time for today.
Next Ill look at Princess Mononoke.