Do you think they really could have been duped?
All Muslims are duped.
But against that is the method, the use of rucksacks does seem to suggest that they could have had a plan for a quick getaway. The time it takes for a train to get from King's Cross to Russell Square is at most a couple of minutes. So it would be possible to get on the train leave the bag and get off at Russell Square and for it to go off before anyone had noticed that it was unattended and alerted staff or stopped the train. The other two stations were reached by a line going out of King's Cross but there would be no way of telling how quickly one could get there so timers would not be able to be set to detonate at a particular station. As the fourth bomber on the bus found out you can't even rely on the tube working which is why their plan for London to be burning 'north south east and west' became more north, east, west and a couple of hundred yards down the road. And there were reports of the fourth bomber fiddling with the timer which suggests that they thought they had some control over the bombs. So I suppose it's possible they were duped but I find it hard to believe that all four could be fooled even if one or two were.