For starters I grew up in this area. My parents home is only 4 miles from the crime scene, and I went to WRHS so I'm very very familiar with what this community is like as I've lived it. However Leominster kids don't really associate with WRHS regional kids barely at all. VM went to Bancroft which would have given her an in with more kids from the wachusett school district. (Holden, Paxton, Princeton, Rutland, Sterling). There are some neighboring towns that are very close knit and interimingled, and some neighboring towns to Princeton, that despite abutting Princeton aren't really part of that community and you don't see a lot of intermingled amongst the towns or their children, beyond maybe people from those towns using wachusett mountain. (Westminster, Leominster, hubbardston). The only reason maybe you'd intermingle with people from these other towns at the mountain is if you worked at the mountain with them (which was a common HS job).
Secondly as an only child myself, and again as one who grew up here, and spent 5 years myself living in NYC....I just can't really agree with your observations. Yes I'm very close to my parents, BUT the only reason I came home biweekly at any point during my time in NYC was to see a guy I was seeing back home. So personally despite being very close to my parents, I was also a strong independent women, living in an amazing city, with a killer job at a high profile fashion house, and trying to figure out who I was all on my own...leaving that environment every 2 weeks to visit my parents would have been more like stepping back then moving forward. (That said I still talked to my mom in the phone everyday).
I think either a guy, or I think the crohns issues are the two most likely reasons she came home frequently. Some on also mentioned it could be if she had issues with her roommates in the city....which also seems much more likely then just because she was an only child, that would be an unusual reason for someone her age to come home.
Had I had a roommate in NYC who was 27 and left biweekly just to visit family, that would have very much seemed like a strange thing for a 27 year old peer to do, at least without a good reason, like a parent having cancer. Let's put it this way, amongst a peer group in NYC, at that age, if one of us left that often, the rest of us definelty would have talked about how odd it was that they left so often and probably cracked jokes about them needing to "cut the cord". It definelty wouldn't be typical behavior. Again I say all of that from the perspective of in many ways walking in similar shoes as VM (i.e. Only child, grew up in the same area, also relocated to NYC for a job in my 20's).