but assume that if an engine is lost, descent to this height would be a (relatively) gentle business, whereas an emergency descent due to loss of cabin pressure would be a much more rapid affair.
Might be, might not be..if you have lost cabin pressure and structural integrity is in doubt you might contemplate a relatively gentle descent, rather than diving height off at high speed.
As for comments about R/T calls ... having had a quick listen to the archived feeds provided by others earlier in the thread I personally had ended up assuming some of the early calls that one would perhaps have expected to have been made when this all unravelled approaching top of climb were missing from the record. Certainly some of the exchanges you can hear are remarkably low key and will be interesting to see the transcripts.