PB,
My thoughts:
But as i said earlier, surely this is more a question of why the pilots took so long to realise things weren't right?
I pondered this because that impression is given in the program, it suggests they were pondering and musing on the problem. However that can't have been the case, they must of had significantly less than two minutes from capture and mere seconds from the DME crosscheck to a go-around decision, they must have acted pretty smartly to correlate the information and make the decision in a very high workload phase.
Use ALL tools available to you, why would you do otherwise?
Probably high workload is a reason why not. Introducing additional and unnecessary gadgets into the scan is probably going to slow the scan and thus hinder rather than help situational awareness. GPS distance will probably different from the DME distance and so will not provide a reliable glideslope check. The DME/glideslope crosscheck is on the Jepp briefing strip and is the position and height correlation to use.