Mortgage payment is approx $1800.

The Heloc can also pose as a debt obstacle since its an open end mortgage a.k.a credit card.a mortgage is also set up over 30 years so it offers a lower rate and payment. 

I offer an online payment portal to submit rent payments via ACH or CC - they "couldn't figure out how to use it" so I set us up to accept Zelle payments.

This reduces your housing payment and helps you build equity. 

Repayment options can be more flexible like in my case I do everything on accrual this really helps flippers when they dont have to make monthly payments.

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.

Then the rents would cover that payment plus the mortgage with cash left over.

In my experience that's not actually the case at least in legitimate evictions for non-payment of rent or breach of another part of the lease.  

Find a down payment assistance program and see what that looks like. 

Your son is responsible for making payments on time or ruining this seller's credit.