In the last years, the rent amount on a BRRRR property has become the defining factor for the loan amount.

I would say first define WHY you want to invest in real estate.

I read over a sample personal umbrella insurance policy and it says that they dont cover injury due to business property, and that business property is defined as residential property rented, or held for rental, to others.

DEFINITELY have a defined lease agreement with set terms. 

The National Flood Insurance Program defines a flood as two or more acres or two or more acres.

The first thing you'll want to do is define the zoning and what the allowed uses are for that property.

Also, come resale time, the duplex would have a proven track record of producing income and has definable value, whereas a single family home is largely dependent on current market conditions and home buyer emotions and buyer pool at that given time

Before you try to build your team, I would recommend: 1.Define your goal - is it long terms passive or do you have more time to say, flip? 

Define your investment goals, including target market, desired cash flow, investment timeline, and risk tolerance.

The first thing you'll want to do is define the zoning and what the allowed uses are for that property.