This may be difficult for investing, but core materials should have some authority behind them.

And from what I can tell, I will need to get a letter of authorization from the borrowers to discuss the mortgage (which I am certain the borrowers would gladly sign).

Does your local housing authority have a home ownership voucher program?  

To say 6 months PITI is required for investment doesn't consider any nuances like LTV and FICO; the Fannie matrix is your authority here.

Cheaper than a crystal ball and much more predictable.Just like author/investor John Schaub says, 'You can't fix stupid', I suggest, 'You can't fix cheap'.

Hypothetically, they would be treated as trespassers and dealt with accordingly by the authorities.

I wish I could, but that Department has the authority to subpoena for the production of documents.

I had deeds and authorizations from the borrowers, but it's possible HSBC stepped into liability territory by negotiating and accepting a payoff from a successor-in-interest (me).

If they are doing something illegal you should contact the authorities.

One thing that happens in some Housing Authorities is that they want some of their lease paperwork to be used - you might want to look into that and see if there is some angle there to work in your favor.