I am a flipper and I have borrowed money from a private lender, who I paid loan origination points to as well as mortgage interest.

Please excuse my ignorance. 1) In an attempt to learn from folks who have done this, what is the best way to finance the gut rehab of the existing property and the new construction of a duplex?

Unless you have a solid plan in place to sell or refi the property within a year.A HELOC on your existing rental would prob still be the best bet, and you may just have to keep calling around. 

I like your househack strategy, just might lower my target to an existing in the $250k range.   

There are several key differences and considerations to keep in mind when investing in a larger building.First and foremost, financing a larger apartment building will likely require a commercial loan, as opposed to a residential mortgage.

Even if a judgement isn't collected through the sheriff's office by executing on the judgement, if the tenant wants to buy a house in that county before the judgement expires (5 years in my state), they will have to pay the judgement otherwise they won't be able to get a mortgage. 

If the latter, and if I continued to purchase mortgage loans or otherwise lend money, is there a definitive point at which I can no longer reasonably argue that I am not in the business of lending money?

That gives you a very short timeline to identify a new property after selling the existing property.

As much as I hate he anti landlord movement, I am starting I see why it has come to existence.

Should I open a business bank account under my LLC  to make mortgage payments and collect rent even though the property is in my personal name?