HF and other comms
HF is still HF which because of the characteristics of that frequency range there are inherent limitations in bandwidth. Calculating the range of an HF frequency is still at best a swag (scientific wild ass guess) because there are so many factors that influence HF propogation. SSB and some of the other modulation techniques do offer advantages but usually these advantages are limited.
WRT the missing AF flight I have to wonder how the data package was transmitted/received. I would expect via SATCOM because HF would not be reliable nor would it have much bandwidth. The lack of bandwidth means you either send a message with fewer characters (not what I would expect for an aircraft status message) or you have a longer transmission time.
Best to all and sympathies to those whose loved ones were on the flight or associated with the flight in any way.
Jon