Look, we're missing an essential point here. Forget all the fancy [or primitive] nav gear and nav techniques. I know exactly what the crew were going to do - I've done it many times myself. The plan was to conduct a VFR transit. You use the nav kit to take the hassle out of navigation, timings, etc, but you use the Mk 1 eyeball to make the decisions. The Mull was shrouded in mist - big deal, been there, done that, dozens of times [specifically the Mull - V-Force, flying low level up the West Coast or through the middle] - but I've continued up the coast using the eyeball to avoid hitting the ground. It's called low flying techniques. It doesn't matter that 500 feet above you there is a solid layer of cloud. If, subsequently, the weather threatens to close in on you, you make the decision to either re-route via an area of better weather [spotted by the Mk1 eyeball], or you climb to safety altitude.
We DON'T KNOW why the crew didn't do what they must have done a squillion times - SOMETHING WENT WRONG THAT PREVENTED THEM FROM STOPPING THE AIRCRAFT HITTING THE GROUND.
Sorry to shout, but I am getting increasingly frustrated by people muddying the waters and forgetting the fundimental principle - the first person to die in a crash is the pilot, and self preservation is the strongest instinct for survival. No one will ever convince me that this flight deck had any control over events that led to the crash.