I think, it's a very good choice.
You can stay inside, or - if the weather is fine - outside on "Alter Markt".
But if you don't like this pub you can cross "Alter Markt" and in 50ft. distance you can enter another pub, for example:
Maximilian's, Brinkhoff's No.1, Alter Markt, Stades, Gänsemarkt, Café Central, Cotton's, Thüringer.
And in the middle you can join the "mob" ).
And when you want to go to the stadium, you can use the stations "Stadtgarten" or "Rheinoldikirche", each station 200 meters away from "Alter Markt".
You take U45 to "Stadion" or "Westfalenhallen" or U46 to "Westfalenhallen".
But, when the weather is fine you should make a little walk ( 25 - 30 minutes ) along "Hohe Strasse" directly to the stadium.
Perhaps you will meet some BVB-supporters and enter together one of the pubs on the way.
How do you think the away fans will be received? I have tickets in the home end, hope all will be easy going !