This won't work for urban areas or non-primary residences, and would have to be single unit, but if you'd consider doing something like a live-in flip you could look at USDA:RD loans and then refi to hold or just sell.

Has anyone used an all in one on their primary residence (or not used it and are willing to share why)?

We're closing in a couple weeks on a 5-room motel in the Tahoe area that also has a 4 bed / 2.5 bath / 3000 sqft SFR "manager's unit" that will be our primary residence. 

The city has made ADUs very easy to permit if it’s your primary but I don’t believe an investor would be able to add one with a separate meter. 

We planning on using our primary residences equity to make more purchases with a HELOC or HE loan.

Did you buy your primary home plus house hacking it?

* I will be using this line of credit for the down payment on another investment property/home improvements to my primary residence.

The other suggestion would be to pull cash out of another property if you have equity primary home or other.  

If I were to BRRRR a quadplex, when it comes to refinancing, would I be able to structure the loan as my primary residence (house hack it).

Good job.Rather than respond point-by-point, I will give you thoughts on one topic and maybe later posters can take on different topics.Step 1 is pay off your primary's mortgage.