Why can't I select the entire 3D structure and run the wind load generator?
Because it is impossible to write a good working algoritme for this. The algoritme should not only be able to distinguish all different floor plans/ roofs shapes, but it should also be able to make a meaningfull interpretation of it in order to generate the wind loads. For that reason BuildSoft has descided to keep the wind (and snow) generator 2D. This offers the most flexible solution.
Do I always need 24 sub load cases?
No. The number of required sub load cases depends on the shape of the roof and the number of internal pressure coefficients. The document at the end of this article explains which cases you need. If you don't feel like figuring out which cases you need, you can always generate all 24 cases. But for 3D structures like the one in the video, its worth verifying which cases you need as the calculation time increases when you have more load cases.
Will the wind generator also run for trusses, lattice structures, scaffoldings, billboards, .. ?
No. The wind generator as seen in the video is developed for buildings. As an alterantive you could enter the terrain parameters and apply a wind load on a group of bars (the selected bars must lie in the same plane). But this method implies that you manually look up the pressure coefficients and enter them in Diamonds.