I have massively messed up Hearts of Stone. I think my best bet is to re-download the DLC for that add-on. I found one (other) forum suggestion to delete the DLC folder and "just download all DLC again". That seems scary! It might reach into other parts of the code that are fine. I just want to be able to restart Hearts of Stone. Any experiences on how to do that?

Why would you need to re-download? Rather, what do you mean by "massively messed up Hearts of Stone"? If it is a problem with the quest, re-downloading is meaningless. If you messed up the quest, you need to revert to a save prior to when you made the mistake/s. You need to clarify exactly what you mean.

If you don't have a saved game from before you started the quest Evils Soft First Touch.
You can just start a new game and select Hearts Of Stone Only.
You will start with a new level 32 Geralt.

I'm so grateful to you for your patience with my thrashing. I discovered MY mistakes and was able to get the game back on track. It proved to be that I had a filter active on my Alchemy formulas and since the Cleansing Mixture didn't have all ingredients present, it was not visible under the filter! When I undid the filter, there it was. So I went charging off and got the ingredients and dashed into town....but nothing happened, no talking guards..OF COURSE! I hadn't "created" the potion. Then I couldn't find the @##@..ing potion in my inventory. OF COURSE it was in my Quest items.
This was the literal comedy of errors. Having found my Mixture, everything proceeded smoothly. Now I'm trying to work through the Borsodi vault. With my skill, poor Geralt will probably be hung from the rafters.

Thought you might enjoy seeing how all this came out. Hoist a cool one for me!

Good to see you got it sorted.