What the Bandai Namco Support representative said is exactly what you need to do, if you wish to play the DLC on a US game, using a German account.
The Playstation Support representative probably misunderstood your question.
will my German account with all of my saved data be able to download/use the DLC's as well?
Use, yes. Download, no. This may be what the Playstation Support rep misunderstood.
Since the game's and DLC's regions must match, you must buy a US DLC for your US game. The game won't recognize the DLC otherwise.
An account can only buy (and download) games/DLCs from its own region. Therefore you need a US PSN Account to buy and download a US DLC. There is no getting around that. Even if you buy a download code in retail (e.g. GameStop), you still need a US account to redeem the code and download the game/DLC.
Since the PS4 is region-free, it can play games/DLCs from any regions. This applies to accounts as well. You can play games/DLCs from any region, using accounts from any regions.
Because of points 1 and 2, your German account must access the DLC from your US account. To do that, your US account must be on the same PS4, and the PS4 must be registered as the "Primary PS4" for the US account.
Things to note:
You can't remove the US account from your PS4. If you do, your German account will no longer be able to access the DLC, even if the DLC is still on the PS4.
The same is true if you unregister your PS4 as Primary PS4 of your US account (e.g. you sell your US account to someone, and that someone wants to register his own PS4 instead).