Excellent questions!
DxO PhotoLab Elite 8 allows toggling noise reduction and sharpening features on or off separately with each image, but I’m not aware of an easy way to toggle NR off in Pure RAW 4. Somebody else may know an easy way to do it. In PR4 you can set varying levels of NR.
Both DxO PL8 and PR4 are designed to not introduce artifacts if NR is applied to an image that doesn’t need it, or if NR is applied at a higher level than needed for an image. That doesn’t mean you can’t find a rare image that breaks the design, but it’s not a common problem. For more specific discussion, see this forum thread: https://forum.dxo.com/t/what-level-of-noise-reduction-do-you-use