To answer your second question first, it is not possible to dynamically change the column offset while using dynamic parameters and actions. It is possible to create arrays using the ARRAY command and then dynamically adjust their column offset using constraints and formulae. It's a fairly complicated procedure.
When using the dynamic array action there are a couple things to understand. The first new object in an array will come into existence when the linear parameter the array is associated with is equal to 2X the column off set. I tend to think of it that the first object (the one that is being arrayed) exists at 1, and the first arrayed object (second total) comes into existed at 2, etc... Because of this, it is often beneficial to use multiple linear parameters (one for each arrayed object) and set their basepoint at the length that works best based on when the new objects will be created. You can then make the linear parameters that are associated with the array actions so they are chained and add another linear parameter that has a stretch action in order to control the chained linear/arrays. See screencast.
