To connect our house to the apartment above our garage or "Additional Dwelling Unit (ADU) we had to have pipes excavated underground to ensure that the garage apartment could connect to sewer/water from our house.

Why not take all the money you would spend on the line of credit each month and add it to your mortgage as a principle curtailment.

Thanks for the additional insights.

If that's 3 houses at $100k each with a cash flow of $200/month each, that would net you an additional $600/month in income after expenses. 

The issue that has popped up is that when speaking to an insurance agent, he mentioned that we would need separate insurance policies for each of the units (which is not a big deal) but then an additional policy for the Condo Association.Has anyone had experience with buying Condo Association insurance?

I do make additional payments on the higher interest ones. 

Additionally, the fiancée has only really agreed to flipping where our neighbors do not live on top, or directly on side of us.

Additional info, I am a business owner, very busy with my business, no time to handle flips or that kind of thing, just looking for solid investment / cash flow, and need tax benefits.  

@Tom M.If the property is owned by the self-directed IRA, the self-directed IRA can later get a non-recourse loan for investing in additional real-estate.

In addition, if they find evidence of previous coverage, they will use that value to issue insurance.