> First two episodes were, and then viewership tanked.

That's simply factually untrue. The Nielsen weekly figures were successively 1253, 1203, 988, 977, 966, 988, 1137 and 570 million minutes. Hardly "tanking" (especially considering that the first two episodes dropped at the same time, so the first week can be expected to have more minutes watched). But I realize this sub constructs its own reality.

HotD was definitely in the Nielsen charts, see here.

In the published Nielsen charts, RoP had 8082 million minutes watched, vs. 8091 for HotD. However, RoP was shorter than HotD (557 vs 618 minutes), so assuming every viewer watched the whole thing, that translates to 14.5 million vs. 13.1 million viewers.