While I do not disagree that all of the above deserve attention, it still cannot explain how an experienced crew (or even an inexperienced crew) allowed their airspeed to bleed off to just over 100 kts
I've been dispatched a number of times in a 767 with autothrottle inop. Even though the need for vigilance in modes like VS was discussed, I've still seen people get caught out and get a little slow before they've realised they had to catch the speed manually. Especially if the autopilot is in, every part of you is expecting the autothrottle to push the thrust up when the speed comes back, even if you've pre briefed.
The method at my airline - 744/767/737 - is if you handfly on approach, take the auto throttle out, so everything is back to basics and makes sense and you're not expecting anything to happen for you. Not sure about the 777, but maybe there is some merit in this line of thinking for that type, too.