Rationalizing this chord progression (I7-iii-IV)

Question

Trying to expand my harmonic palette here, and came across this song.

Chorus of Stevie Wonder's "Don't You Worry 'Bout a Thing" (in E♭ minor, but temporary shift to relative major, G♭):

G♭       G♭7     B♭m7    C♭maj7
A♭add2   D♭9sus  G♭maj7  B♭(ALT)

I'm pretty sure I understand the rest of the song's harmony: stepping into the relative major, then we end up on the IV chord, then II-V-I, and V-i back to the relative minor for the verse. The way we got to the IV chord, though, is a mystery to me. I'm confused about the particular sequence [G♭7, B♭m, C♭], however. I can see the I7 (G♭7) chord moving to the IV chord (C♭), but the iii chord in-between is stumping me.

I'm picking up that the iii-IV move sounds nice, so from that angle I can see why the iii goes to IV, but the I7 going to the iii sounds kind of strange.

I'm currently thinking of the chords as a I7-IV movement with a passing chord in-between, but that seems strange to me, because the harmonic rhythm has this chord land on a strong part of the phrase.

I guess my other possibility would be a deceptive resolution to the iii chord, followed by some sort of delayed resolution to the IV, where I feel the song is pulling towards...

What would you guys do to analyse this progression?

(For starters, have I misheard the chords? Is there a better justification for the chords?)

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(And please, let's stay away from "if it sounds good, it is good". You and I both know that; if I were looking for that kind of answer, I wouldn't need this site to tell me about it.)

Answer 1

Here's my take by listening to what the clip sounds like. I just assume you transcribed them correctly enough, except for the G♭7, which I think is better explained as F♭/G♭ or D♭m7/G♭.

• G♭ : tonic
• G♭7 : Instead of "G♭7" it sounds more like F♭/G♭ (or E/Gb or whatever), giving a mixolydian/bluesy feel, implying that it might be going to go to B (or C♭) next, or just toying around with indefinitely held tension like you do in blues. To expand on the idea of the first two chords here, play G♭ - D♭m7 - G♭ - D♭m7 - ... You're supposed to think that the tonic might actually be C♭.
• B♭m7 : This has two feelings and interpretations at the same time. (1) If you believed the "C♭ is tonic" suggestion, then this feels like the start of a B♭m7 - E♭7 - A♭m progression. Think Beatles's Yesterday as "C♭ - B♭m7 - E♭7 - A♭m". (2) But if you didn't fully believe the "C♭ is tonic" suggestion, then it means we abandon the "G♭ - D♭m7" mixolydian/bluesy idea and switch to G♭'s relative minor side E♭m. But not really as strongly as possible ... The stronger version would be to use a proper dominant B♭7. (in which case for interpretation 1 it would mean a secondary dominant, going for B♭7 - E♭7 - A♭m)
• C♭maj7 : another change of mood. After the B♭m this feels like a clarification that the "C♭ is tonic" interpretation was wrong, and now we're at G♭'s subdominant i.e. IV chord.
• A♭add2 : it's just a tweaked II chord. A plain A♭7 would sound like a too obvious secondary dominant going for A♭7 - D♭7 - G♭.
• D♭9sus : dominant chord, with some added stuff to make it a bit more ambivalent.
• G♭maj7 : tonic. We're home.
• B♭(ALT) : it's a tweaked B♭7, a dominant for E♭m, meaning a switch to the dark side.

Answer 2

The progression III - IV is as logical and useful as any other combination of related chords in a key. And especially as III and I7 have 3 common tones it III can - as you mean be considere as a substitution of the tonic, aswell as the VII can be functionally used as a dominant7 without a root tone.

Any degree can be followed by any other and these progressions and combinations existed before the theory of functions.