I'm struggling a little bit on deciding the best agreement to make sure I can get some cash flow, payback the HELOC, and not drive the new mortgage up too much with the Refi out of the HELOC.

Have you refinanced this into a long-term mortgage?

You also cannot use a second mortgage (from HML/private lender) for a 2-4 unit non owner property. 

In both cases to keep things clean for underwriting at the bank I used my HELOC to secure the mortgage. 

With the COVID-era surge in home prices, rapid increases in mortgage rates, and rents not keeping pace, it really has gotten harder to make these deals work.

Finally, always get the credentials to access the mortgage account online and change the email notifications to your address.

I also am very interested in purchasing more STRs, and this is a really low risk way to see if I'm up for it (we have a 2.9% interest rate on our mortgage).My biggest concern is an ongoing plumbing issue we have had.

It may or may not show on a credit report, depends on the servicing and you should have the loan serviced.Failure to include the debt on a future loan application is mortgage fraud, wanna go to jail?

After those sources are RE agents, mortgage brokers, friends and family.  

When that portion was complete, I could then receive draw money which allowed me to pay my utilities, taxes, and mortgages.