Hi mate, both will do fine. If you dont want to think too much about buying tickets and expect to go around town a lot then get the pass else just buy when needed. Not much of a difference in the end.
On gameday it makes the most sense to walk from the event(Old Irish) to Vesterport(2 min walk) and take the train to Østerport station(3 min) and then walk to the stadium(15-20 min). Or get a taxi. And the same way back.
It is. However there will also be metrobusses in this period as an alternative to the metro. But either option is still a lesser option than the metro. I will try to figure out What is the best option and let everybody know.