.............WHY would an IP, even an inexperienced one, not monitor and correct a low airspeed situation?
(Sorry if I'm repeating what has already been stated, I just don't have time to wade back through 2000+ posts.)
As a one-time I.P myself, I can appreciate a situation that is "Supervising" rather than "Instructing". If one instructs, then one tells the student what to do, but in this situation I see it as an experienced pilot who has presumably gone through "instruction" about the B.777 and passed simulator checks, and maybe even other flights, and is now being "supervised", in which case I can also appreciate a need to give the subject enough rope to - not hang himself as in this case - but to recognise and correct his own mistakes, i.e. as the Supevisor not leap in too soon with criticism and condemnation. But in this case of course he left it too late before intervening.
I would sometimes tell my, apparenty experienced, candidates for route supervision, that I wouldn't interfere unless they were going to kill me. Clearly this was a necessity - the interference that is ! - in this case, why it didn't happen is a mystery.
If 'Loss if Face' is considered a factor, surely the I.P. wouldn't have 'lost face', the student would, and why not if he is about to kill everybody ?
I've picked up from a previous post a suggestion that the handling pilot might have had a 'mind-set' that stationary throttles with A/T engaged was OK, because he was recently from the Airbus, and so thinking that the A/T was engaged, as has been reported, stationary throttles on the Boeing that he was now flying might not have alerted him - maybe - but that again begs the question of what the I.P. was thinking ?
Not excusing, just saying.