Which white powders form acids when mixed with water?
From the pH and solubility information alone, I would propose sodium bisulfate: cheap, readily available, safe in the powerfully oxidizing solution. (Possibly also potassium bisulfate.)

One way of manufacturing $\ce{HCl}$ produces sodium bisulfate as a byproduct:
$$\ce{NaCl + H2SO4 -&gt; HCl + NaHSO4}$$ The odor of $\ce{ HCl}$ could be a minor impurity in this mystery acid. The other impurities also suggest it is not a reagent grade product.