OP Summation (for the time being):
Regarding this matter, the embassy website, notes two addresses in PA to which I can mail the SSA-7162 form. I downloaded it, printed it, filled it out and signed it, and scanned it and sent it to my sister in the U.S. She will mail the document certified, return receipt, so that I have documentation.
Further, for redundancy, and to cover all bases. I am in the process of submitting the same to the embassys here. That's messy. The site says that you can mail it. The embassys uses Domex for passport re-newal and I have done that successfully. But when I went to Domex in Sto. Dom. today, they refused to accept the form and said they could not deliver it. That makes no sense. There is a fallback procedure. The embassy site also explains how you can make an appointment to deliver it in person. I completed that procedure and receive an auto response that they will respond in 5 business days (and next Monday is a holiday), so I have documentation of my good faith efforts.
I've done everything I can do, so now I'm waiting.
When I originally received SSA benefits I read through a series of brochures explaining the ins and out of receiving benefits in a foreign country. I opted out of utilizing direct deposit to a bank here. My takeaway was that I would have no issue received my benefits while being an expat in a foreign country, that it was perfectly legitimate. But I don't remember reading anything about the biannual mandatory documentation nor did I ever receive such documents at my official address.