I think the question about the first half of the missed 91 total minutes is....so what happened during that time?
The Denver ARTCC is a big chunk of real estate:
Military Comms Monitoring. HF VHF UHF
about 500nm across on the SW>NE path of this flight (from roughly GCN to LBF and beyond).
One would expect there to have been other handoffs or attempted handoffs in that 45 minutes within Denver's area - unless there was just one controller handling all the high alt. traffic that evening across (conservatively) 200,000 square miles? Heck of a big radar screen!
Certainly there are a lot of "high alt." frequencies assigned to Denver Center.
Was it SOP for a plane to be silent for that distance and time - and no one caught on until the exit?