Rent on a 4 bedroom place may be $5000 monthly ($12,000 in Manhattan); rent it by the bedroom for $1500 per bedroom with access to all common areas.  

Their most common threat is a security deposit dispute.

A famous quote: Do unto others as you would have them do unto you” is commonly known as The Golden Rule.

It’s not as common, though cause it takes a lot more work

This is Occupancy Fraud, a very common type of mortgage fraud. 

I still have clients buying in Vegas, but it's not as common due to cashflows being hard to come by with the traditional 25% down financing.  

I have a few reasons for this. 1) We are both self-employed and our income is inconsistent. 2) We don't want to impact our DTI, because we are planning on buying a house hack within the next year or so.We would be open to going up to 6 units for our first property as long as it's within budget.Now that you have some context, is it possible for a rookie to use a DSCR loan on a duplex or triplex?

But usually for a tenant occupied it's like $90-120 per monthI've noticed a lot of multi-family buildings are pre-WWII housing inventory - are there restrictions or issues common with that (historic districts, crazy high utility costs, etc)?

One just uses the tried and true wholesaling methods and each geographical area will have its unique ways of doing this.With a lot of land deals starting with tax sales. this is common in CA AZ etc etc. currently I am funding deals in WA ID MT CO AZ TX FLA and then some other states.

This is not overly common, but if you have a concern the worst they can say is no. 4.