HOWEVER, I am doing everything I can to ensure the problem is not with the house (as much for my sake as theirs, because it's not going to do me a damned bit of good to have them pay for it if it's going to keep happening).They have turned this incident into a reason NEVER to pay for anything.

Is there anything specific I should be asking/saying to them (maybe to put some pressure on them, but also to ensure that everything will be alright)?

All of which is factual data you can research, which is the basis for my statements to my clients and on biggerpockets. 

I did the 1 month trial for his cash buyer data feed, contacted hundreds of people on the list repeatedly and got nothing.

Using a combination of assessment data, google street view, count GIS mapping information, and old MLS listings I can usually make a fairly quick screening of each property before going to do a drive-by.

Okay so now he's having some trouble again and wanting to come over and discuss how to ensure my wife and I retain the land, we don't have enough money to buy it for cash, or loan, for market value nor a reduced cost for work done already.

They had next to no data available. 12 lots had homes on them (out of 30 available).

The numbers in the tool are as reliable as the data chosen to perform the analysis, and that is up to you, not the software.The tool currently uses Zillow data to suggest a number of comps for you, however you still have do the work of verifying their suitability for your analysis.

Who ensures the accounting of mortgage (principal reduction, interest paid) every month?

Disclaimer: I'm not a data scientist and these calculations are only representative of my own opinions, take them with a fair amount of salt.