1-2) I live in one of the EV studios. I don't participate, but I know they have a lot of events, and they seem popular. I think if you are outgoing and want to build relationships with people in your housing community, you can. Personally, having lived on my own for years, the thought of having a roommate is horrifying, even though I would love to be able to save the money. But you may feel different, and it certainly is easier to get plugged in that way. People that choose to live in the studios are also more likely to be less outgoing or otherwise wanting no distractions while focused on their work, so you might want to consider that.
3) Groceries: there is a newish market just a few minutes walk from the Northeast corner of EV. Not large, but that have pretty good produce options, and reasonably priced. I haven't seen many customers in there, so hopefully they stay open, as there was a very short-lived market in that local the previous year. On campus there is Munger Market that is really convenient for simple stuff, but they don't have a ton. More substantially, within walking distance there is Mollie Stone's on Cal Ave, about 25 minutes from the center of EV, and a little pricey. There is a Trader Joe's in the Town & Country Village, also about a 25 minute walk. You can also reach it via the X line of Stanford free bus service. There is a Whole Foods downtown, which you can get within a few minutes walk of with the X. There are two Safeways by Stanford bus. There is one on Sand Hill Road that you can reach via the SLAC line, but that bus only runs on weekdays from the center of campus. More useful is the Safeway in San Antonio Center in Mountain View, with the Shopping Express goes to. The Shopping Express runs afternoon and evenings on Weekdays, and all day during Weekends (one of the only lines that runs on the weekend or holidays).
If you are living on campus without a car, you can also get a free Zipcar membership. They have a lot of Zipcars on campus, and some are only about $5/hr, so that is also a faster option for getting groceries.
4) I don't know how it is with the students in Biosciences, but most humanities PhD students who live on campus at all seem to generally stay for their first three years. It's hard to find a cheaper place to live without a horrible commute (welcome to Silicon Valley), so if you have to be on campus everyday, it really makes more sense to stay.
Currently, if you move off campus it can be hard to get back on, as you no will no longer have a housing guarantee. They have a lot of new housing opening in a couple of years, though, so this may change.
5) What is the best bang-for-your-buck really depends on what is important to you. If you don't mind having roommates, then by all means save a bunch of money and have roommates. If you just want the nicest housing experience regardless of cost, then Munger is for you. If being closest to Biosciences is most important, then probably Oak Creek is your top choice.