The window must have a net clear openable area of 5.7 square feet (5.0 square feet if ground floor), a minimum openable height of 24 inches, a minimum openable width of 20 inches, and the bottom of the opening must be within 44 inches of the floor.Size: The room must be at least 70 square feet, and no less than 7 feet in any horizontal dimension.Lighting & Ventilation: A natural light source (window) of not less than 8% of the floor area and natural ventilation (openable window/door) of at least 4% of the floor area are required.Heating: The bedroom must have heating capable of maintaining a minimum room temperature of 68°F.Smoke Detector: A smoke detector is required in each sleeping room.While a closet is expected in most bedrooms, the International Residential Code does not actually require a bedroom to have a closet.

You’re spot on, you can potentially use a 3.5% down FHA loan for new financing if income and rents from the other three units are sufficient.

Your primary insurance policy with $300,000 in liability coverage should be sufficient in 99.999% of all lawsuits.5.

It is my understanding that you cannot use the ADU for short term rentals in Los Angeles.https://therealdeal.com/la/2019/10/29/while-you-were-sleeping-la-bans-granny-flats-as-airbnbs/If you make the ADU all electric (like I did on mine), then you don't need to worry about splitting the gas. 

If the equity is sufficient for the down payment and qualifying for the new property there are some excellent HELOC or even fixed rate second mortgage programs available.

I've seen lenders exempt years spent in college as "work history" but the borrower would still need to have sufficient funds/reserves and a solid credit score. 

Sounds like you have sufficient seasoning.

Usually a 1 to 1 or 1.25 is standard..or the $1k needs to be sufficient to cover the PITI of the proposed loan.

Otherwise a typical DSCR loan based on market long term rents should be sufficient to qualify? 

It seems to me that these are sufficiently dissimilar endeavors.