Ah, now the joy of cleaning up. Here's what I'm doing, for anyone in the same predicament
Step one: Delete any DNG's that never stacked up
Step two: Import all my D750 NEF files
Step three: In gallery view mode in the Library module, apply a filter as follows - Choose D750 under Metadata-Camera, plus add a text filter where Filename-Does Not Contain-psd to get just the D750 DNG and NEF files.
Step four: (with filter still on) While still in gallery mode, go thru the NEF files one at a time, deleting them if there is no corresponding DNG file, and applying any Star Color or Flag Attribute that might exist on the DNG file.
Step five: (with filter still on) Go to the Develop module and on every NEF file choose Paste Settings from Previous (thank God there's a keyboard shortcut)
Step six: Go back to Gallery view mode in the Library module (keyboard shortcut 'g'), clear the existing filter and add a text filter Filename-Contains-DNG. Select all and delete.
I know what I'm doing in front of the TV tonight.