Not an economist who has studied these issues, just and armchair intellectual here.I bought a house in my neighborhood and my realtor thought I should airbnb it. 

Does anyone know if you can use a cost segregation studies for STR properties that you may have outside the US (and use it against your US W2 income)?

My advice, don't go for a homerun, a base hit is just fine.. buy something solid and get educated about your market.Its sounds like you are spreading yourself a little thin with all those markets to study. 

There are specific rules for 1031 exchanges including:The replacement property must be identified within 45 days of transferring the relinquished property.The replacement property must be received within 180 days after the transfer of the relinquished property or the due date of the taxpayer's tax return, whichever is earlier.Special rules apply for exchanges between related parties (defined in § 267(b) or 707(b)(1)).Real property located in the United States and real property located outside the United States are not considered property of like kind.Cost Segregation StudiesA Cost Segregation study is an IRS approved federal income tax tool that increases near term cash flow by utilizing shorter recovery periods for depreciation to accelerate return on investment.

You have to research, study and find the indicators on various I.D.'

In any case, I usually do not talk much about my scientific work...

A recent study showed that 80 people are moving to this city every single week, which will continue to increase housing demand.

Which appliances can I include in the study and on what cases?

Study the market.

Being 1.5hrs of NYC gives you a lot of options, even Long Island (which typically has more options for public commute into NYC).Give me a shout if you'd like to chat more, I have a handful of case studies on my site as well from deals I've closed/brokered that match the househacking criteria you're aiming for.