Is there a place inside the park to buy opposing teams Tshirts, etc?

Yes

Yes...there's a big shop

Yes, many places sell opposing team items inside the park.

yes of cours

Yes but y would you want to?

Yes there is

There's a vendor on the street, on the walkway that comes from the football stadium, that also occasionally sells opposing team items. Usually for the more popular teams, and usually later in the season

Yes, but it depends on who the Os are playing.