First, so that you are not mislead, know that the Scheidt and the Scheidemann "Erbarm Dich" verses are pasted together with Audacity, so there are indeed registration changes that you do not hear since I recorded each movement by itself. However, all the changes could be accomplished at the organ with the aid of an assistant or two.
Between the movements of "Erbarm Dich", for example, the pedal loses three stops, and the OW also loses three stops. While the OW introduction is playing, the HW plenum is replaced by the solo combination you hear, and there is plenty of time to do that. I just didn't have that many hands available...
You say it is smaller than you were thinking, but I will say in reply your guess of 25 stops is quite close to the real total. There are indeed a few mistakes in your proposed stop list, but there is much that is correct as well. I still make no comment on how many manuals the organ has.
But perhaps one of these demos will help.
A verset by Carson Cooman written for this organ. It is all of 36 hours old... There are no registratoin changes in this piece. If you can figure this out, it may go some distance to eliminating many organs with similar size and specifications.http://clavmon.ics.cas.cz/ErikMP3/coomanverseti.mp3
In this next demo, the second half of the chorale features the soprano cantus firmus played on the pedal rather than in the manuals as notated. You can hear the tremulant acting on the three lower parts only.http://clavmon.ics.cas.cz/ErikMP3/pachelbelanwasserflussen.mp3
Compare the manual registraton of this chorale with the second half of the Scheidemann Ballett "in F"http://clavmon.ics.cas.cz/ErikMP3/pachelbelvomhimmel.mp3
Finally, a comparison of the two plena of the organ. The only reed stop you will hear is the final pedal notehttp://clavmon.ics.cas.cz/ErikMP3/scheidemannpraeambulumd.mp3
Don't read too much intp the locations of the composers, beyond this being a North German sort of organ where this repertoire works very well (and things I can play with little preparation). But do listen to Carson's piece carefully for many clues,