Was it Gunter Mittag or Hans Fleischer?
I see you modified your question. Besides from the rule not to ask multiple questions, I can tell you that it was not Hans Fleischer either. Hans Fleischer was responsible for the Wartburg design.
And I did a little more research and I think this model, which I found pictured without a proper name, is not really the Trabant 610, but a very closely related design with a name of its own. But it was done by the same designer as the 610.