first of all, are you using the latest svn version ? The Irlicht module of the version 1.04.00 was the first one and it has a fews bugs. You can download the latest version directly from here.
For the delay, it is normal, you got to tune the system so that it follows the camera. If their is too much delay, you can increase the radius of the ring used to generate the rain (it is not a ring but a aabox in the previous rain demo in 1.04), increase the gravity so that the drops falls faster, lower the altitude of the ring, increase the density of the fog so that we see less far... it is all a matter of tuning.
Another way of not having a delay at all, is making the entire system relative to the camera. You can enable that when constructing an irrsystem (by setting worldTransformed to false). In that case, when moving the irrsystem like any other node, the whole particles will move with it. So instead of moving only the zone, move the entire system. It can work for for example car games where the camera is moving very fast but with camera moving more slower, their will be no delay but the effect wont be that good (as the drops are moving with the camra).
For you other issues try to use the new version and tell me if you still got problems.