careful,
>> Were they examples of a "new" cong C formed from the now defunct congs A and B? Or maybe a cong D formed from old congs A, B, and C?
They totally weren't
>> How can one know such details? New names could reflect the changing realities of territorial adjustments or just a broader geographical name that encompasses more area. Who knows?
Did you open the first link in the OP? Notices of renaming are published separately.
>> Could they be at least partly because of immigration, a huge issue in Germany?
Or of Watchtower's expansion into new fields. Yes. >95℅ of created congregations are foreign- or sign-language.