Some gurus tell practicioners of sub-to to hide transaction from the lender by saying the lender doesn't care as long as the payments are being made.

The two existing LTR units (on my family’s property) were built 5–8 years ago at a cost of about $80,000 per unit, so I’m estimating a 20% increase in costs for the new builds.My plan is to use the equity in the land as the down payment for the construction loan, which I expect will make closing costs the primary upfront expense.

If you do have to move because of a job, you would be safest (in my opinion, i am not giving legal advice or financial advice), i would sell that property to ensure that you did not just purchase with the goal of moving out early and taking advantage of lower rates and lower down payments for purchasing your home as a primary residence. 

I have a W-2 Job, capital saved up for a down payment (or two), and a buy box based on my affordability.

Explain your situation and see if there are payment plans or hardship programs available.The BiggerPockets forums have threads on tax liens and foreclosure prevention: https://www.biggerpockets.com/forums/41/topics/766025-surplus-funds-unpaid-taxes-in-foreclosure-saleAn experienced real estate attorney can advise you on your legal rights and explore potential solutions.Have you spoken with your local tax authority yet?

How will the partnership cash flow cover the HELOC payments if interest rates increase?

Quote from @Brad Roche: The other thing that is great for the FHA 203K loan (not sure about the Homestyle loan) is that you can kick the payments out to 6 months of ownership (so close today and make your first payment July 12th 2025) if you are fixing up the home.

Unlike the Bay Area, at least from my experience in Sacramento, i don't think the difference in down-payment from a duplex and tri-plex is that significant.

There is no requirement to use them as mentors for doing the rehab projects as described in the horror story above, but they do their own underwriting so they will be investing themselves to assure the deal makes sense and is successful to assure they get their payments and interest. 

NONE of the money raised from the auction of a 2nd goes toward the payment of the 1st (that is why they call it "subject to the first")4.