CADENCE

I do see your point, Serickso... and both of the examples in my original post could be seen as 'in key' in the context of modulation - D to G would be authentic, and C to G plagal.  But in that analysis, neither would be a half cadence, except in relation to the key signature.

If we're in Cm, the F# within the D chord would create a leading tone to G, a common enough device among composers.  The D chord would still be 'out of key', though... with G as the dominant, we're in either C (no accidentals) or Cm (Bb, Eb, Ab, with B and A optionally natural) and neither has the requisite F# for a D chord.

In context within compositions, I agree with your statement... the F# as a leading tone has more melodic than harmonic significance.  Analyzing a chord progression absent the melody, though, the D is outside either C or Cm.

By the way, welcome to the forums, and I look forward to interesting discussions!

Tom