Our entire global system is built on a mountain of debt... consumer, corporate, federal, etc.

I'm going to assume they are letting you use your schedule C/E from your other rental for income verification OR you simply make enough otherwise to cover any debt to income ratio needs.  

You mentioned on hand reserves and that is an additional amount that is also raised upfront. 3 months of projected operating expenses and debt payments is typical for stable properties.

Paint a picture that selling to you eliminates much of that headache.

You will pay a higher interest rate from a lender than you will on your LOC.Neither of these funding resources are designed for long term debt, you'll want to have an exit strategy.

To answer your question, your Debt to Income is what is evaluated when purchasing multiple properties.

A 15-year will bump up your debt service by $400-450/month.

My debt to income ratio is too high, because they are holding the debt from the loan used to purchase the Airbnb against me.

Financing the debt at below market rates would ensure that it'll make business sense for you not to refinance that debt for quite some time, keeping his payments secure. 

I am having similar issue with this also.Add on to that, I am looking at my income and debt ratio also.I got some SFR on mortgage and some fully paid off and they are all on my name.