JFK is not acceptable as a planned alternate field due to the phenomenal levels of rush hour traffic that field experiences. Add in to this that JFK currently has one runway out of operation for reconstruction, and is likely to have a shared destiny to EWR in terms of weather and related delays.
Pretty much the same as the authorities frown on folks filing Heathrow as a planned alternate for anything landing in the UK.
Bradley and Newburgh/Stewart are both reasonable choices for diverts in the New York area, in terms of runway length and ground handling ability for heavy jets, this despite being "secondary" airports, and possibly not having full-time CBP presence.
Diverting to BOS may seem sensible, but BOS could have already been backed up with existing diverts from the NYC area. As for IAD, that tends to share it's fate with EWR and PHL during summer thunderstorms.
Quite why there wasn't ground air conditioning provided, I don't know. I'd be very surprised if the ground handler at BDL didn't have a suitable ground air conditioning unit.
From reports, VS seem to be particularly poor at activating what other airlines would consider standard contingencies during ground delays.
"Excuses" about VS getting "no service until the airport know they will get paid", while it might be true for transient aircraft, is just utter BS for airlines such as VS. VS have accounts with large global handlers such as Servisair and Swissport for exactly this reason.
There are things that can be done to ease ground delays, and if these reports are to be believed, they were not done. Typical false optimism of "Oh, we'll be going soon.", "Oh, we'll be getting off soon." Why do us Brits practice such optimistic denial?

Mike