I think I'd like to tackle each of these one at a time as standalone games. Is the best way to do 3 separate installs:
DS
DS+
SW

Or can it all be installed into one game folder and versions "activated" as desired. I ask since as I have it installed RoG seems to permanently modify DS (no way to play vanilla DS), and I can't find a way to switch out of SW once it's installed.

Any pointers here?

Don't worry of installation paths etc... Instal everything, with the default path (so all in one game folder).
You can select the exact version you want, but only when you start a new game (you can't change during a game)(if I'm not wrong).
When you want to start a game (when you have RoG and SW installed), you see two boxes with red crosses : this means that if you start now, it'll be Vanilla. If you click on RofG red cross, the red cross will turn in a green tick : RoG activated ! And same for SW. (keep in mind that you can play RoG or SW : they can't be combined)

I made screenshots to help you if you need :)
Hope this helps and answers your interrogations !

awesome thanks! I'll install it all to a single install.

You're welcome :)

Hmmm... to get ROG to appear as an option I needed to install DS v210020 instead of v210022. With the updated version of DS I couldn't activate or un-activate ROG.

EDIT: I installed v210022 over the game install and everything is working fine. No patch version appears anywhere so not 100% v22 "took". But it all seems right.

I... don't understand why you've this problem...
Have you downloaded all the latest installers ?
If you have any doubt : uninstall everything without keeping the saves, then download latest installers of game + RoG + SW, and install them in this order (DS, RoG and finally SW). It should be ok.
(but if it's fine now, maybe you shouldn't bother more ;))

Sure enough - my version of SW was v20 not v22. I'm good now. But really thanks for showing how to load the specific games - that threw me for a loop. I though it must be handled in the mod menu...

Perfect ! :D
Ahah, no problem, it's always a pleasure to help :)