Pregame chow before Sunday's game
There are lots of pub-type option in Federal Hill, the closest walkable neighborhood to the stadium. Mother's is a good choice, they have great burgers and wings. Ropewalk also has good pub food. Keep in mind that parking will be insane and you'll have to use the garage or find a meter, all the permit parking is stadium-restricted and if you park there you'll get a $75 ticket at least. Also all the bars will be crazy crowded. But it is easily walkable from the stadium.
Most other places do seem to be closed until 5 on Sundays (I just checked Matsuri and Thai Arroy's websites).
That's a bit of a tough one. There are a lot of great places in the area, but many are either closed on Sunday or don't open that early.
Mother's is a bar/restaurant at Charles and West that many like. For some reason I've never made it in there even though I live in the neighborhood. There are several bars in that area that serve food but I'm not sure when they all open -- my fave is Dog Pub, but they don't open until 5. Ryleigh's Oyster Bar is good for oysters, but I'm lukewarm on the rest of the menu. Beach Bum on Light Street makes gigantic sandwiches and seems to be under new management, which was a good thing. Sobo Cafe is just west of Charles on Cross Street. A lot of people like it, but I'm not a big fan.
I had never really thought of it before, but the noon-4 pm window on Sundays really isn't a good time to be grabbing food in Federal Hill.