RecentFile works only right at launch. As soon as there's some scene it isn't active any more (which of course makes sense).
Would it be possible to use the same Recent list for Import too? Some "Import Recent"?
Think a label like "DiscardUndoHistory" or "ClearUndoHistory" would be more self explaining (but maybe to long). ATM this clears the history immediately. Think some confirmation dialog here, explaining that you can't undo previous things any more would not be that bad.
@ "d" on DropSolids:
Just fine. - Not sure whether that's a "small improvement": Maybe its doable to add some "AutoDropSpacing"? Meaning some slider threshold which drops a duplicate automatically after the MESHMIX part was moved a geodesic distance (set by the AutoDropSpacing threshold in % of the part's current diameter-similar to Spacing in SCULPT/Surface) from the last drop (the first one done via "D")? Zero = NoAutoDrop.