Short Answer: Outside of a hockey-interested circles people will likely know VERY little to nothing about NHL or even their local DEL team. In a hotspot they will know about their DEL team and maybe a tidbit about NHL, especially like knowing Draisaitl or Seidenberg.
Some passionate hockey fans & people who actually play inline or ice hockey will know very much, watch NHL games, have a favorite NHL team, and so on.
Oversharing:
Again I am super biased. I play in a men's hockey league here, so interact with people who play the sport. I live in a big city which is also a hockey hotspot. Generally speaking Cologne and Mannheim have big pro arenas and something like a cult following of hockey fans & good youth programs. Note that Dusseldorf is only 40km away.
Looking at the attendance rankings you may get an idea where pro hockey is popular in Europe. In Germany: Berlin, Cologne, Mannheim, & Dusseldorf.
IIHF - European attendance ranking
I have seen Oiler jerseys randomly in the city (note that I have an eye for this, but it is really surreal).
At a local DEL game I would expect to see more NHL jerseys and run into people who know NHL Teams and Players for sure. The Oilers played here in 2018 and in 1991 in a mini-tournament called Epson Cup in Dusseldorf (an older guy I skate with refereed one game and showed me the scoresheet in person!)
Subjectively I noticed a lot of German NHL fans seem to like the Ducks (maybe from the Mighty Ducks Movie?), otherwise Original 6 esp. Boston (Seidenberg?), Tampa, or in general the California Teams.
Oddly I ran into a few Scottish & British hockey fans that are super passionate and know the game. One English guy loves DEL and travels often to Germany to see games. He has NHL Jerseys, and I think calls games on the radio in his local arena!
Average sports fan will not know McDavid in Germany, no.