I heard one expert say that a cockpit struggle (e.g., the captain over the copilot) could well explain the rise to 45k and an increase to that height would not compromise the pressure in the cabin.
You can easily kill the passengers by dumping cabin pressure and keeping your plane at altitude for a period that exceeds their emergency oxygen supply. You are correct; the 777 can fly at 45,000 feet for extended periods, but at that altitude, without a pressurized cabin or supplemental oxygen, you’re toast in a few minutes.
BTW, the emergency O2 supply for the pilots is much greater, and they could also access the “walk-around” bottles once the passengers and flight attendants were subdued or dead.