Yes a gradient is better so they can choose the best area to be in at any given time so just wetting one half is ok, plus it will move along the granules at the bottom anyway.
Depends on the species you have
Camponotus vary a lot. Look at their data on the site here and use that as a rough guide, it's down to experience really so you need to learn yourself per colony, not too dry and not too wet (hard to explain how much that is exactly haha)