hi! first off, don't worry that you're overthinking! it's so good to be prepared and it's great that you're already looking into your options!
i'm a second year at UoY and know a bit about each of the questions you've asked so i'll try to answer them as best as i can!
1) in first year i was in Vanbrugh catered accommodation (in Eric Milner B - not being used anymore but is very closee to James!). at the start of the year i was very nervous about making friends so sometimes did end up going to dinner by myself (breakfast by yourself is much more common because you start your day at different times to your friends!). this was honestly fine because there was always loads of space and tables to sit at! by a few weeks into first term i started going to dinner with some flatmates and it was such a great bonding experience! i actually live with these friends now and go together with one of them so it definitely brings you closer together! as the year went on we'd tend to all just meet at around 6pm unless someone was busy and all walk over to eat together! plus it's so convenient especially if you've had a long day and just want to have dinner as soon as you can! i wouldn't worry though if you did have to go by yourself - there will always be someone there to sit with if you really want to!
2) i studied biology (which is over the bridge in James college) and being based just next to James in Eric Milner was definitely convenient as it took 5 mins to get to workshops etc! i don't think you'd feel claustrophobic as you can always take a walk around the lakes and see the ducks if you wanted! i will say though that just because you're studying psychology, it doesn't mean that all of your contact hours will be in the psychology building! i've had in person sessions in the biology, maths, psychology, and even english buildings so you won't feel trapped into a certain area of campus!
3) i have a few friends from James and it definitely is one of the sporty colleges but being in any other college doesn't stop you from doing sports!! the west campus sports building is next to James which is very convenient but it's available for anyone from any college to use!
i hope this helps!