Apparently, the usual way InCD works is that when you write data to a DVD-RAM disc, it needs to update the file allocation table after writing the actual data to show the changes made. I've heard complaints that if you add data to a disc that already has data on it and InCD runs into a problem updating the file allocation table, the whole disc will end up blank. This and other bugs seem to be the reasons why InCD is not recommended by most people I've asked about it.
Panasonic's DVD-RAM drivers (DVDForm is part of that) work fine for me, and I have never tried InCD myself. However, I can vouch for the fact that even with a bad write (e.g. dirt on the disc), Panasonic's DVD-RAM drivers return your disc to the state it was in before you attempted that write.
These and other reasons are why Panasonic's DVD-RAM drivers are the recommended software for DVD-RAM writing.
I have never tried Sonic DLA either as my LG writers all came shipped with Nero.
4a)Not necessarily, it depends on your DVD player. Most DVD players have no problem with either type. My advice is to buy one piece of each type and try first.
a)Nero Vision is meant to be easy to use, but it doesn't necessarily do the best job of converting video to DVD/VCD format. There have been reports of out of sync audio when some files were converted to DVD format with Nero Vision. It is better to use a separate video conversion tool to process the video files, then burn with Nero later. The best place to look for help for this sort of thing is probably the guides section of http://www.videohelp.com.