You told us that 400-500 mg of elderberry in capsule form was equal to the antiviral effects of 50 ml pomegranate juice. Grimed Elderberry 21,900 mg – X15 Powder from Amazon also has Vitamin C 100mg, Vitamin D3 50 mcg, Zinc 50 mg, Quercetin 100mg, Bromelain 100 mg, CoQ10 100 mg, Nattokinase 100 mg, Ashwagandha 30 mg, Turmeric 20 mg, Black Pepper 20 mg, in addition to a concentrated extract equal to 18,000 mg of elderberries. This is also economical, as it costs $14.95 for 150 servings, a 5 month supply. Does this source deliver quality ingredients that are safe to take daily, and is this supplement as or more potent in antiviral activity compared to 50 mL of pomegranate juice?

You need not worry, this is a reliable source dedicated to quality and meeting their specs, so to speak. So the supplement can be relied on to have safety and quality ingredients. It is, as you suspected, more potent than the pomegranate and by a significant margin.