Ah but when Cobb tampered with the totem they were already very deep into dream state, where a change to the environment could have a profound effect. It's not

Ah but when Cobb tampered with the totem they were already very deep into dream state, where a change to the environment could have a profound effect. It's not a oneway camera because the subconscious constantly reacts to things that they do to the environment/projections, no matter what level they're on. Furtheremore Cobb doesn't so much change the environment (which I agree would be too easy, although he had to go to the trouble of finding the safe first), as reactivate something which Mal had chosen to bury and forget. If Mal merely had to look at the spinning top for Inception to work, then he could have just dragged her in there and made here open the safe, and when they woke up she could have seen that the top no longer spun. What he did was change a constant such that she had no idea why she now doubted reality (on every level). In the other dreams the safes contained pieces of information, but Mal's safe was different as the contents were something with mechanical/programmatic agency, a moving part if you will.