I wouldn't doubt that diffusion happened before the Fall.
The 2nd Law describes decrease in order (tho who is to say that a shared
scent is not more "orderly" than a confined one?).
Decay is not the only process that decreases "orderliness", although it is
very prominent in our fallen decaying world.
The curse (decrease in the blessing of God) allowed decay, increasing
disorder vastly, with less thorough regenerative cycles.
Thermodynamics, including 2nd Law, was real from the beginning. The main
thing is that it precludes any overall increase in order.