A. chionitis seems a reasonable assumption, even though the orange/yellow terminal tufts on the abdomen are not visible in these photos. Did you see them? There are no black forewing terminal marking which are present in apparently one sex of A. semichroa, although that species is shown on Bold Systems 4 to have primarily a Tasmanian ie. southern distribution (- we have one record on CNM here in Canberra, see the species list).
Once the flying moth landed, neither of them moved. Even though I got quite close while taking photos from different angles, they both kept their wings firmly closed.
Doesn't the lower specimen have the subterminal black forewing mark often found on semiochrea? That species is quite widespread, not just Tasmania. I'll post a new comment on the report currently identified as A, chionits shortly.
Yes, a second look at Part 2 MoV confirms that in males of A. semiochroa the black subterminal forewing mark is highly variable and is sometimes reduced to a mere dot.
No, the upper moth was already sitting there when I first saw it, which was after the lower one flew in to join it. The lower part of the cocoon was hidden by the two moths, which didn't move while I was there.