I found no indication of somebody owning DC1TC between Horst and me, so it’s very likely that I’m the second owner of that call.
And to answer part 3 of the challenge: These days there is no indication of geographical location for German callsigns anymore.
Sources I used:
https://www.dl4no.de/thema/rufzeich.htm
https://www.darc.de/einsteiger/amateurfunkausbildung/#c152704
https://archive.org/details/dlarc
http://qrzcq.me/do5ssb/BNetzA/Calls/
www.dl4no.deRufzeichen und Lizenzklassen in der Bundesrepublik Deutschland