No. DNG files are RAW- why keep NEF+RAW (doubles storage needed!) But you must have the DNGs backed up somewhere else!a. Do keep both your NEF/RAW files and the DNG duplicates or just the DNG
No. Lightroom contains the 'Treatment' in its Catalog where you can have multiple Virtual copies or Snapshots that do not occupy more storage space. But you must back-up the Catalog- preferably on an external drive with the DNG image back-ups.b. Once you've given it the lightroom treatment do then save another duplicate DNG file?
Yes-it can be avoided! Only keep any Photoshop PSDs (complete with layers) in the same folder as originals, and never keep JPGs- they can be exported from Lightroom at any time in the future when needed. Remember you have those multiple Virtual Copies and all the output presets (which you can define) to output JPGs in any size, format, renamed, etc. (and no extra storage needed for the extra JPGs!)c. Not all photo's need photoshopping but saving a PSD and then finally a JPG seems to be a step that can't really be avoided...
One more hint- Make the Photoshop PSD file a 'final' image edit- Why? If you re-work a PSD in Lightroom develop, Lightroom will first flatten the layers of the PSD so rendering all the Photoshop editing work non-returnable.