probably, the foil liner of the mylar bag would reflect UV light back into the water which would help kill bugs
if there’s a layer of plastic between the foil and the water, the plastic might absorb any UV, in that case the foil wouldn’t make any difference
the boundary between the water and plastic, foil, or air will reflect UV depending on the angle of the light and the indices of refraction. Basic physics class problem.
probably best to assume no UV is reflected back
I don’t think there’s a good answer to your question about foil liner. Won’t hurt though