His home ground was Windsor Park when he played for Linfield. Having played at the old Casement Park I wholeheartedly agree with you that it should have been rebuilt and used for the Euros but I am not sure it would have been used as a home ground for Northern Ireland as it was to be handed over to the GAA. Personally I think the biggest missed opportunity was when it was decided not to build a new stadium at the Maze or Dargan Crescent for the GAA, rugby and the IFA with a rail link to Belfast but that is our politicians.
Graham would have played his football at training facilities miles away from Windsor Park. Which is a good job, as he probably would have been traumatised playing in front of those Linfield fans. He's better off over here where only his talent will matter.
As for Casement Park: there's no reason that couldn't have been an alternative for Windsor Park for NI games. Yes, there'd be issues initially. But then so too are there issues for people from Nationalist areas (should they have wished to attend) who have had to make those compromises for decades and decades.
If barriers are to be broken down over there and walls to tumble then it's around cultural stuff like this providing a lead.