Some music theoryh

 That makes perfect sense. 

With the progression: 

G  →  Bm  →  Am7

V     ?      vi

the Bm is acting primarily as a linear/passing chord. Notice the bass line: 

G → B → A

and the chord tones: 

G    = G  B  D

Bm   = B  D  F#

Am7  = A  C  E  G

The notes B and D are common to G and Bm, so the Bm sounds like a smooth extension of the G harmony before settling into Am7. 

Another way to hear it is as a temporary iii chord in G major: 

G major scale:

G  A  B  C  D  E  F#

iii = Bm

ii  = Am

So for a moment the harmony leans toward G-major colors before returning to the song's overall key center. 

What's especially interesting is that you've already identified several "color" chords: 

Cm

D♭maj7

Fm6

Bm

This is very characteristic of sophisticated Motown and R&B writing. Rather than staying strictly in C major, the song moves freely between: 

• C major harmony
  

• borrowed chords from C minor (Cm, Fm6)
  

• chromatic color chords (D♭maj7)
  

• smooth voice-leading chords (Bm)
  

If I saw G → Bm → Am7 in a chart, I would probably think: 

"The Bm is there because it sounds beautiful between G and Am7."

The voice leading is more important than assigning it an elaborate Roman numeral. 

You can think of it as: 

G → Bm → Am7

V   vii/??  vi

but in practice I'd simply call it a chromatic mediant/passing chord connecting G and Am7. 

That's one of the things that makes songs like All I Do Is Think of You so rich—the harmony is driven by elegant voice leading and color, not just traditional functional harmony.