I've looked at a few of their deals now and they're averaging close to 26% cash on cash returns.

Can anyone provide insight on the average cost of insurance for Airbnbs with pools in the Boston area?

I think your best approach is to pretend you are the investor yourself, calculate build cost (mortgage with interest) + Maint / cap expenses/ management fees according to your zoning and then compare that to the average rent in that neighborhood.

But it is not an easy home to fill given the rental rate is above average for the neighborhood (but well below average on a $ per sq ft basis). 

Are there any good resources online to be able to get an idea of average monthly/yearly expenses like utilities, etc. for a given zip or city?

From what I’ve read in the Wall Street Journal, average rents in Providence are up about 15% over the last two years, which could work in your favor.

If you can’t quantify risk and don’t understand the mechanics of “weighted” average of expected return and range of returns (standard deviation) then you are at a severe disadvantage and should consider a competent investment / financial manager for your investment capital.  

Just calculate the average price/sqft and use this, but this isn't good because some out of date also get sold, and the likesFinally, how closely do investors follow the 70% rule?

Conversely keep in mind the prevailing "average" for Property Management fee is 7%+.

Percentage is a rule of thumb, so whether you get a 100 a month or 1000 a month, your maintenance is going to average around a fixed number not a percentage of your rent.