<3. Passengers have reported there was no oxygen supply to their masks. Then how did the passenger oxygen masks deploy?>
It is quite possible that they didn't hear (or chose to ignore) the bits in the safety briefing about :
'Pull the mask down to turn on the oxygen supply'
and
'Note that the mask does not inflate'
If they didn't pull them down firmly, no oxygen.
If they didn't see the transparent 'balloon' bit inflating, perceived lack of oxygen !