I don't suppose the clicking has anything to do with the DRM on the audio file from iTunes. Did you try another type of audio?
I don't think you can combine project files (.nvc) in NeroVision, but if the projects are done, maybe you could use something else like virtual dub to put them on one DVD. I have no experience in this area, so this is just a thought.
If you have them in .vob form, you might be able to add them to NeroVision and burn them together, but I don't know if the slideshow part would work.