I think you are correct about a lot of places being named after Dan. He did leave quite a trail of place names, but I don't know about Brandenburg. There are too many other solid names like the several rivers leaving the Black Sea starting with or sounding like Dan to try and make that stretch.
The longer names may have been phrases. Like maybe "river Dan crossed" may have become Riodaneber to use as a funny example using a couple of different languages and throwing the DN in there. There's too many to be a coincidence after Eastern Europe. I believe that's what happened. Plus at first notice there wasn't a vowel in the names such as Dneiper, etc., but as the languages modernized, more vowels were used to where the names sound modern as they went west. Makes sense to me, if anyone doesn't want to believe, then they won't.