Hiya thehac,
1. RSD will be a stand alone product, but it will not be WYSIWYG in the sense of drag and drop and you're done. It will be drag and drop and now position and style your element using the tools provided in the program for adjusting padding, margin, color, etc. Although it will not be the simple place it and go, it will be extremely powerful.
RSD will fully complete your site (from what I've seen and heard here on the forums and emails), so that you shouldn't have to tweak all that much outside of the program. It will incorporate the export feature that RLMP has right now as well so when you're done you can export the site and edit the code to your hearts content same as we do now with RLMP. You'll have full reign of the code outside of the program.
Now there may be "inside the program" html tools that I am unaware of so maybe someone else can enlighten us both on that part.
2. RSD will not require the use of RLMP in any way that I'm aware of. The hype to get it now is for your benefit to start learning the ins and outs of the program while you wait, and to be honest you can complete one heck of a lot of your site with RLMP right now.
What it WILL do though is allow you to create a site now in RLMP and then import that site into RSD when it's released. That is one of the other high selling points for RLMP right now, you can get a very large jump on the learning curve before RSD is released and you can still use RLMP after that to whip up quick site mockups for a client's viewing should the need arise. Other than that no, it's not required.
The program is also not only being made available only to RLMP users, they are only getting it sooner than everyone else as far as I know. Special discounts usually go out to those that have purchased certain software items as well and I believe that's the plan here if I recall the emails correctly if you've purchased RLMP.
3. Being on a Mac myself I am pretty sure they plan to release it for both Windows and Mac, hopefully we won't have to wait for it while everyone else is using it O.O
Maybe someone else will come enlighten us a bit more on the things I'm not sure about