If it's in an area with high appreciation I would try to extract the equity via a HELOC or cash out first.

I've explored refi and heloc, but came up empty as I have no chance of qualifying income.  

I've self-financed deals my deals, financed them with a HELOC and used third party lenders as well.

Make sure you know the value the ADU will add to the property before building the ADU. 2) the financing on an ADU is typically far worse than for initial investment property acquisition or is often not leveraged by the ADU (HELOC, cash out refi, etc).

Have you looked into getting a HELOC on the home now that you have a lot more equity in it?

Otherwise I’d keep he current property and get a heloc on it or my primary.

You can leverage your HELOC and cash to cover the down payment, the initial rehab draws, and holding costs. 

Another question is… Which option would be better to cover the missing mortgage payments: a HELOC or forbearance?

Make sure you know the value the ADU will add to the property before building the ADU. 2) the financing on an ADU is typically far worse than for initial investment property acquisition or is often not leveraged by the ADU (HELOC, cash out refi, etc).

Most banks won’t allow for a DSCR underwrite for HELOCs.