I think the biggest benefit of the AIA system really shakes out to be the effective & clear communication of the guidelines and procedures for construction management, which tend to be much more muddied in the residential world.

Have I just discovered the secret to getting rich in real estate?

Effort is still required to communicate with the Craigslist advertisers, but having software to lift the deals out of Craigslist would surely be worth $1,000. 

Neither unit has paid rent and has blocked us from communications.

Communicate with your lender if cash flow is tight.

A certified letter is good because they sign for it, acknowledging receipt, any other forms of written communication you can get to document that they were notified of the hazard probably also helps. 

Just make sure whomever you work with is halfway competent and communicates well, the latter is the most important.

I would keep and organize all of your communication around this issue, including gathering pictures of the unit and the area in question that was repaired.

Armando being the first one to really blow this up became VERY VERY rich..

However, you are correct, their CoC returns will never make you rich in the near future.