There are certainly investments you can make to increase the principal in the net 5-7 years.  

Here’s my question: Should I use my w-2 income to pay down a lot of principal and re-try to obtain that HELOC?

It cash flows gross $400 max and about another $450 goes to principal each month.  

All this fancy mumbo fha guideline gumbo means is that your projected market rents assuming all units, if it were rented Times 75% need to be greater than what your monthly payment is  (PITIA = principal/interest/taxes/insurance/assessments monthly), other wise FHA will not lend at that loan amount so common solutions to this when rents are too low in your local market are to put more money down payment which defeats the whole purpose of low 3.5% down FHA doesnt it?

Best option is to either sell the house or have her buy you out at today's market value PLUS return the money you've already invested (ie down payment, paying down the mortgage principal).

I've included an example below to help illustrate this.So different lenders have different rates (which do vary even for DSCR loans) but these are factors they all consider.See example below:DSCR < 1Principal + Interest = $1,700Taxes = $350, Insurance = $100, Association Dues = $50Total PITIA = $2200Rent = $2000DSCR = Rent/PITIA = 2000/2200 = 0.91Since the DSCR is 0.91, we know the expenses are greater than the income of the property.DSCR >1Principal + Interest = $1,500Taxes = $250, Insurance = $100, Association Dues = $25Total PITIA = $1875 Rent = $2300DSCR = Rent/PITIA = 2300/1875 = 1.23If a purchase, you also generally need reserves / savings to show you have 3-6 month payments of PITIA (principal / interest (mortgage payment), property taxes and insurance and HOA (if applicable).

. — renege on principal and interest payments.

The transaction allows investors to increase the liquidity and diversification of their real estate investments while deferring costly capital gains and depreciation recapture taxes that may result from the sale of a property.Benefits: REITs also can provide the same ongoing benefits of real estate ownership including income, depreciation tax shelter, principal pay down, and appreciation.

No down payment means you invest for nothing, take advantage of principal paydown and tax advantages and maximize your BAH to do so.

Usually you make a few points at closing and then the interest-only on the loan monthly with the principal coming due at maturity.