OK ;- ).
1) Vray 2Sided Material was just Vray's translucency. You didn't have to populate the second slot with anything. If you didn't, it was exactly the same as Corona's translucency.
2) My setup is
absolutely identical to one linked from Corona's help. I also use single translucency map and value. This simulates what happens "inside" the leaf and the color inside is the same.
3) I added different
Diffuse and
Glossiness maps to back and front part of leaf. It has nothing to do with translucency. Leaves look different from front side and back side. Nothing to do with translucency or color variation among leaves.
Here is photography: Front part is darker, more saturated and glossier/shinier.
4) Outside of this, it's possible to do lot more advanced stuff like using CoronaMultiMaterial to randomize any parameter (Diffuse, Translucency, Gloss, etc..) to make each leaf look different (younger, older,...).
5) It's possible to go even more advanced than that by using Vertex Color Gradient generated by Tree pograms (like SpeedTree or GrowFX), that you will multiply on top of (Diffuse, Translucency or Gloss) to make sure than brighter younger leaves are on end of trees, and older are inside.
But the basic setup is always the same and there are no different methods. Just one.