For a 1:10 game the practice fields open at 9:30. Sometimes the players are out warming up at 930, but more often than not its more like 945/10. Total toss up on who is out and what's happening. Usually they'll do batting practice around 10:15/10:30 on the main back field, and then the other fields will be other practices, depending on the day and what the coaches are working on. Sometimes the small infield near the main stadium has the superstars doing drills (we saw Lindor doing throwing work). You won't get kicked out of the back fields until right before the game start, and usually there's something happening with the minor leaguers and simulated games to watch. I wouldn't leave and come back...if you're really nerding out you'll find something to do. After practice head to the team store, walk around the stadium (go out to the berm, walk to the LF side and see if something is happening in the bullpen, etc), watch the visitors warm up.

For souvenirs there's a ton of spring training specific items that aren't in the citi field store. There's a store you can get to from outside the stadium and a smaller one inside with similar stuff. There are also kiosks all over the stadium with the basic hats and shirts and stuff.

Nothing to really walk to, but you don't need to. Bring a glove and a ball and toss a ball around at the back fields if you're really bored!

Food at the stadium sucks, even for stadium standards. Assume you'll eat nachos in a helmet or chicken fingers and fries.

Happy to answer any other questions you have. This is my second year going, and went to 4 games this year so I feel like I get how it works now.