I can speak for the G4. You can conduct flight operations without putting fixes or approaches in the FMS. But it s pain in the tail to do so and you lose much of the automation. It doesn't take too long to put the required information in the system to make it work. Not having it increases the workload exponentially.
The Honeywell FMS system in the G4 can be mislead by false glideslope readings. Also it is possible they didn't switch over to green needles. Which also could have given them false readings. When things go wrong it, the more important is it to slow down and take your time.
I would in no way consider India an aviation flag of convenience. I have dealt with them, They could give Britain lessons on bureaucracy. Bureaucracy in and of itself does not make anyone safer. In fact it can and does make it less safe as the bureaucracy tends to keep operators busy jumping through make believe hoops to take care of the real safety issues.