1) Since DC owns intellectual rights to the new WB production of DS, that means that he can control scripts and/or has final say so. Right?
That completely depends on the deal he's made with John Wells and Mark Verheiden. My guess is that he doesn't have the 100% final say on anything. Why would Wells and Verheiden be involved if he did? And I'm strongly suspecting that the involvement of those two men is what got the WB's interest. On his own, DC had been shopping DS around for over 10 years without much to show for it.
2) Does DC get any money from MPI for the DS conversion from VHS to DVD format? I believe that you said the actors who interview do.
I've never read any concrete info on this, but I doubt he'd get money only for the simple reason that the episodes are being released on DVD. However, he most probably worked out some sort of deal for the show's release to home video, and the DVDs would most probably be included in that.
3) What exactly does an executive producer do anyway?
That varies from EP to EP. Some are producers in name only and have no involvement in the day to day operations, while others have involvement in every aspect of a show. DC isn't the type to be the former. Normally he's the latter - but with Wells and Verheiden's involvement in the WB DS, who knows where he falls within it?
4) How long can DC control intellectual rights to DS and can he pass those rights on to an heir?
He probably controls them for life. But he can assign them to anyone else whenever he wants to, which would include passing them along upon his death.