lemme, your speculation increasingly relies on assumptions of what the crew were thinking; their plan at the time, not ours with hindsight.
For future safety it's good question why did an aircraft with a reasonable chance of achieving some form of landing, ended up on a hill side; particularly when the industry recalls at least two previous events with a successful outcome (Gimli, VIE), involving higher tech aircraft, in a similar technical scenarios, and of the same certification basis.
The degree of success depends on being on/off an airport, the distance vs altitude available, weather, and terrain. The latter two in this event were unfavourable.
A pilot might be more interested in conserving altitude, not descending early to meet some objective, particularly if this is defined by range information which the crew may not be aware of (no DME display). Also, the speculative 3-4D navigation assumed that the crew had accurate altitude information; this required resetting the StBy altimeter scale; the alternative is ILS GS. Is there any evidence that ATC broadcast the airfield datum.
I doubt that the crew would recall the glide ratios for configuration; I would not recall 15:1*, but for other reasons 6:1 is remembered, 1nm/1000 ft (note range requirement). Thus a plan might involve stretching the glide until committed to the final configuration selection.
Emergency navigation information is shown overlaid on the StBy attitude gyro (not IRS) as a two axis cross pointer. Without care this display could be interpreted as a FD command.
With a VOR tuned the lateral axis would provide directions to the ground station; I cannot recall what the vertical axis displays, but if shown and followed as a FD command, then a null reading could result in unmonitored flight into terrain. AFAIR there is no range information.
With an ILS tuned then both axis would be valid, but the aircraft would have to maintain flight at or above the GS. If the display was used as a FD command with the aircraft closing from above or being on the GS, then there would be a point at which the aircraft would be unable to maintain both speed and GS if fully configured (but 15:1 clean or low flap, low wt vs high alt, might be an interesting challenge).
* 20 years on type, including steep approaches.
The 146/RJ aircraft types have never to date, suffered 'deep stall'. The 'straight' wing design has a conventional stall which is forcefully assisted with a stick push for the manual controls.