TLDR: As an improviser, viewing the ii as a sustained V7 is a much better way of approaching the 2-5 in a 2-5-1. Outline the minor chord is a flavor, both a dominant in relation to the tonic.
Sometimes we must challange the building blocks of our own systems to gain some insight, which I believe I have and wish to share. The diatonic 2-5-1 is not what we are taught.
We all know the descending tritone shell in a circle of fifth, where the chords all have a dominant quality. Its a clear voice leading movement creating and resolving tension along with the bass that fall a fifth. In my view this is what we should have as a basis for analysis of a 2-5-1.
For a 2-5-1 to Cmajor we should think D7 G7 C7. What we do in a regular diatonic 2-5-1 is that we cram these chords into a major scale, but this also alters their functions. For a 2-5-1 to C we apply the C major as the scale we cram this movement into
D7 -> (F# -> F) -> Dm7
G7 fits as is
C7 -> (Bb -> C) -> CM7
This in turns alter the defining voice leading movement. From
(F#, C) -> (F, B) -> (E, Bb)
To
(F, C) -> (F, B) -> (E, B)
This modifies the sound and in my opinion the function of for the 2 in particularly.
We change the voicing leading by loosing movement to having one semitone movement per chord change plus a decending fifth in bass, C -> B, F -> E.
This also turns the 2 from having a tritone with a tension (C, F#) to sounding more resolved with a fifth/fourth (C, F). So when I listen to the movement 2-5, I dont hear a tension pulling the 2 to the 5 as I do when the 2 is dominant, but the it feels natural.
In my opinion the function of the 2 has changed completey here, the only important movement is C -> B. Example, rootless 25 voicing
Dm9 (F A C E)
G13 (F B C E)
What Dm9 here does it acting like a suspended dominant. We literally have
G13sus/D -> G13.
In other words we have a suspended dominant to a regular dominant over a falling fifth in bass movement. This could change your view on how to play over 2-5 as a 5sus - 5 which simplies everything. And it is your choice to sustain a dominant, hence its 5 - 5 - 1.
If you wish you can of course outline the minor chord but IMO this a flavour but should NOT be taught to be the standard way! It teaches the 2-5 the wrong way.
If you wish to break the diatonicism, playing D7 G7 will outline the actually 5-1 to the dominant (V/V to V for the pedantic), but Dm G7 is merely a sustained dominant which is an optional flavour.
What does this mean for the minor 251?
Dm7-5 has the same origin as the Dm, D7, cramed into another scale, harmonic minor, becoming D F Ab C. It looses one of its voice leading movements but retains the other one (C -> B). By the same logic it acts the same as the Dm to a G7, its
G7sus(-9)/D -> G7-9 -> Cm6 (or mM7)
And again sustaining is optional, so playing G phrygian donimant language over both is perfectly fine.
Another consequence of this that we reveal that the important notes defining this chord are Ab and C (not the third). The Dm7-5 is in fact functionally Fm6/D, which is why a voicing like D G Ab C works equally well. And why if you wish to distinguish the 2 you play Fmelodic minor.(