Due to the Systemic Sclerosis, and the medications you are on, I assume you are being medically managed by a rheumatologist at least every 3 months. Have your past CBCs been normal, or is the elevated RDW a new onset? It sounds like your cardiologist was happy with the baseline echo, with a recommended follow up echo in 5 years. Repeat studies are appropriate to monitor changes in cardiac structure or function when there are changes in the clinical status of the patient, or when disease progression is otherwise suspected. After clinical evaluation it seems your cardiologist feels certain more invasive tests have more risk than benefit. It is extremely important to eat healthy and maintain a good exercise routine.
As to your question: "I want to know if taking amlodipine in conjunction with quinapril for a number of years, for reasons other than heart problems, would basically nullify the value of the NT-ProBNP test and whether there could still be something going on." That would best be answered by the doctor who you regularly follow up with, most likely your rheumatologist. They have full knowledge of your clinical history, physical exam, previous labs, and any diagnostic imaging that has been performed.
None of us can diagnose you over the internet. Given your medical history and medication management, I would encourage you to address your concerns with the medical provider monitoring your care. Of course if you are having chest pain, please proceed directly to the ER. I wish you all the best.