I've got about $525K in equity in the property, but lenders have told me that I cannot get a HELOC to access that equity for additional purchases because the duplex was financed as an investment - they have said TX law dictates that HELOC can only be used on primary residences.

In some cases, federal tax credits like those for low-income housing or economic development might also apply if the project aligns with certain goals.Instead of buying the land outright, developers often lease it from the tribe, which can reduce upfront costs and sometimes come with favorable lease terms.

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?

Although I've considered a future line of credit against the property as my best long term option, I would need to pay down the remaining balance first.

I need to go talk to some credit unions..  

In addition, we pull our own comps and use Lightning Docs to generate our loan documents.

After talking to support (who were actually quite responsive and helpful), they told me I had to publish the ad with only one photo first and then re-post the ad with additional photos later to force the first photo to be first.

Was able to get a meeting with the seller and negotiated using evolving knowledge of construction and deal analysis in addition to discovering seller motivation through direct contact.

do not use a line of credit for a down payment. 

Credit cards have the same limitation on the amount you can borrow and will likely end up costing more than private lenders.