1. Yeah those games are pseudo-3D, that is isometric. If the view rotates like in C&C Generals/Zero Hour for example, 3D becomes true. Even though that isometric does not require full 3D, you can still use a 3D engine like Away3D to help you out. It is always easier to think in 3D as we are used to a 3D reality and maybe abstracting to an isometric/grid based system is more complicated. Away3D's triangle caching system is ideal for isometric views since you only render things whose visual representation changes in regard to the camera. A good example of this is Intel's ITM3, which is done with Away3D and uses massive poly counts (given that its Flash). The rendered result is isometric, but you can still design elements in the scene in 3D, so you can have things rotate and not actually need impostors for everything... Also, sorting is done automatically. Using tri-caching efficiently for something like this would certainly require a few neurons, but what you can do is pretty powerful.
2. Another advantage of using Away3D is that you could use its elevation map features, so a good use of this would do all the work for you regarding this aspect. 3. An isometric view would involve, from the use-a-3d-engine point of view, placing the camera at a given position and probably using an isometric lens, but the objects in your scene could still be axis aligned, so it wouldn't be too hard to develop an XZ grid system and very easily detect when moving objects have entered a "cell" in this grid. hth!
