1) There is no "campus" as such, the buildings are rather scattered across town. But generally Fridd Site is reasonably quiet mostly, peaceful and people are very friendly. The SU have started some "halls competitions" this year which is an attempt to break the "keep to themselves" you mentioned, but most other sites are much more communal. This is probably to do with the halls on Fridd being really new and all the doors are closing automatically, while on the other sites there are bigger corridors and people can keep their doors open/unlocked, so it is much easier to get to know people beyond your immediate neighbours/opposites. Around the buildings too, it's mostly quiet and friendly, but I can only speak for Main Arts as all my lectures are around there. On Normal Site sometimes you see swarms of people in pjs going to lectures in the morning and that, so I think it's all a bit more "easy going" up there..
2) The SU is great. They are making a real effort for students and are great campaigners for our rights and also for our student experience/social life. There are many events that different societies put on and I think they have plans to make more general public calendars of these available, so if you're stuck at home one night you can just go and see what's on.. They did also run a nightclub which has closed now due to the bulding being demolished, but the new one should be ready for fresher's week this year, so yaaay!
3) See (1) mostly, I'm not aware of different reps for halls, mostly only for sites. Though people say that those staying on College Road (Rathbone and Garth) smell weird (because of the corkboard used to construct the halls), and Reichel has had a bit of a bad reputation the last years but they are renovating it currently, so it should be all good next year.
4) Pretty much any pub that's not so hidden away. In Upper Bangor that's mainly Rascals, the Belle Vue, Paddies and The Menai, and on high street there are numerous pubs and bars, mostly all of them regularly full of students. There also is Bar Uno on Fridd Site, though I feel that it is quite empty there most of the time and most people I know don't really like the place (loud, lots of empty space). I think it kind of depends on what degree you do as well, linguistics/english language students are often to be found in the Belle Vue, I think you meet the biology/science lot in The Menai regularly..
Hope that helps!