I had scarlet fever as a child. I'm now 46 and all rapid strep tests result in faint positive-culture comes back neg. Is the SF somehow to blame?

No. The rapid test samples mucous/germs in your throat. It has no way of being influenced by your past medical history.Your throat mucous can never yield that data. The "faint" false positive has been proven negative enough times that it suggests a degree of unreliability for the test used, or how it is being done.
No. All tests, especially rapid tests like Strep screen have false positives and that is likely to be the case. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, low fat milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.