Plenty of buses do go to the Amex and without the match day traffic it shouldn't be too hard to get there and back 25,23,29, all go straight to the station and the 49 drops you off nearby in Moulsecomb. Timetables here
Yes. I'll be getting a lift or the bus back. Shame about the trains for those coming from further afield than Brighton itself. Was never going to miss it though