A Sukhoi superjet 100 is missing

[Annex14]

MORA - MSA

Inside a 25 NM circle south of the airport MSA is set at 6900 ft MSL
Outside that circle the MORA for the sector with Mt. Salak and Mt. Gede is 11900 ft MSL. The taller Mt. Gede is the reference, all the mountains are outside the 25 NM circle
source of information: Mr. Gerry Soejatman report and maps.
stonevalley
Thanks for the pictures they give a good impression of the terrain and also about parts of the weather that can change very rapidly - as was reported -
RetiredF4
I am willing to follow your arguments if the flight rules were changed from IFR to VFR somewhere during the flight, however that is not reported so far to have happened. May be it turns up as soon as the CVR is deciphered.

But as you stated too, untill different facts become known, one has to believe that this was an IFR flight untill it´s very last second. The descend clearance requested sounds more familiar to an en route descend request to position ones flight for an intermediate approach altitude towards an approach to land.
If this proofes correct, there is only the possibility the crew descended into clouds ahead on their intended flight path and never saw Mt. Salak, anticipating that ATC would not clear them below MSA or into a mountain.
Intentionally I leave the terrain warning complex outside. that is a totally different field.
Remains one additional question: Why cleared ATC the flight below MORA and MSA?? Doing so, why not first request the pilot to cancel IFR and if he did advising him to stay VMC and clear of terrain (stay clear of clouds and maintain own separation to the ground - used to be standard phrase as I remember)
If not the pilots, they (ATC) - as locals - should know their airspace and the limits. So evaluating the radar recordings - if any available - and correlating Radar positions against RT-transmissions and CVR may deliver some valuable clues.