1) In the ObserverCamera class what is the depth returned by getDepth?

It's the depth of the virtual visitor in the plan.

I understand that the elevation is given by Z, right?

Yes

2) On the Camera.Lens enumeration does PINHOLE correspond to "depth of field"?

It's the NORMAL lens. A PINHOLE camera matches a very small hole in a black box, i.e. no depth of field because of a very small aperture.

3) For the "depth of field" lens is there a way to change the default focus distance of 2.5m so you can fine tune which objects will get blurred or not for being out of focus?

From version 4, you can change that distance by setting the com.eteks.sweethome3d.j3d.PhotoRenderer.lowQuality.normalLens.focusDistance or com.eteks.sweethome3d.j3d.PhotoRenderer.highQuality.normalLens.focusDistance System properties (i.e. these changes will act as if you set global variables).
See also com/eteks/sweethome3d/j3d/PhotoRenderer.properties to get the full list of the properties you can set for SunFlow renderer.

4) For the function "setVisualProperty" of the different classes, what are the default available visual properties if any?

Looking for the references to this method in Eclipse would have given you the answer. It's called in AbstractPhotoController, HomeController, PlanController and VideoController and their matching view class. All the property keys used in Sweet Home 3D core classes start by "com.eteks.sweethome3d.", so just don't use this prefix to avoid any collision.