sadpole:
1. Where/when did they do it? (It is not in the transcript)
It is not recorded on CVR
2. Why would someone do it without loudly communicating it to others?
All approach looks very non standard, like improvisation
3. Why would something like that be done without any discussion?
There is significant lack of in cockpit crew communication in this case / not existing 4 members crew procedure and lack of expirience of them/
4. If the the autopilot was not on the H-hold mode, what mode was it in?
Klich told pitch stabilization mode
5. How did they get so low without monitoring instruments?
Ony heavens know - improvisatins, stress?
6. Why would the dangerous altitude sound 0.3[s] after the 2nd issues the goaway command? Probability of coincidence is almost NONE here.
I do not understand your question - why it could not be a coincidence