1) You can bypass the Jamma harness for the video, just wire in an extra connector to the monitor. I can probably get a picture of how I did it in my cab in a couple of days, if you want. Make sure that you unplug the Jamma harness from a game, otherwise the picture goes funky.
2)I've tried the DC2Jamma way and failed, it turns out not every SCART adapter for Dreamcast is the same(Some have composite video, some composite sync). I'm going to try the VGA Box hack next.
3)From what I can tell that adapter doesn't convert any signals, only puts them to the right pin on the Scart cable. However, I might be wrong, I just found out that a S-Video to composite adapter is just a capacitor placed on the right pins! If you do try the adapter, tell us if it works.