To be honest, I'd stay in Bochum till late afternoon, as there are far more bars than Gelsenkirchen
30 minutes from Bochum HBF to Gelenkirchen HBF- then it's a matchday tram to the stadium.
However, it's probably a 10-15 minute walk to the away end
Plenty of return trains, but normally you have to change at Essen (which isn't as difficult as it sounds)
Your matchday ticket will cover most of the journey, but I recommend that you visit the Bochum DB Information Office early afternoon, (they'll print-off your travel itinerary with platforms at Essen etc)
.