I normally play Resehvsky's variation against the Nimzo, but it looked like Paul was trying to learn the black side for tournament play, so I decided to play the classical variation which I think he's more likely to encounter OTB.
4... O-O 5. a3 Bb4xc3 6. Qc2xc3 Nf6e4
More usual is 6 ... b6. Now the game starts looking more like a Dutch, but without a dark-squared bishop for black.
7. Qc3c2 f5 8. g3
Probably suspect, but I like systems with a fianchetto on the K-side. They lead to positions that my feeble mind can understand ;)
8... b6 9. Bf1g2 Bc8b7 10. Ng1f3 Qd8f6
Black gets some initiative out of this, but with white still having a dark-squared bishop he will find it difficult to capitalize on it. It makes more sense to continue to put pressure on e4. I was more concerned that black was going to drop the N back to f6 and open the e4 square for the bishop, for example 10 ... Nf6 11.O-O Be4 followed by ... Qe8 and white is getting in trouble.
11. O-O a5 12. e3
White wants to get rid of the excellent black N on e4. This move protects the d-pawn so that Nd2 can be played, which forces an exchange of minor pieces. This move also weakens the light squares, but with black's queen on f6 and the light-square bishops swapped off there's no obvious way for black to exploit them. If the black Q were on e8 or g6, this move could be fatal.
12... d6 13. b3 Nb8d7 14. Nf3d2 Ne4xd2
Too obliging, IMO; a better option might be 14 ... Qg6, with the intent to recapture on e4 with the f-pawn after 15.Nxe4, allowing black to pressure the weak f3 square. So white would probably have to play 15.f3, preserving black's light-squared bishop.
15. Bc1xd2 Bb7xg2 16. Kg1xg2 e5
Black now has the initiative. However, white's weaknesses are on the light squares, and black is lacking in pieces that can exploit those, so his attack should not succeed. This is why I think that black's Qf6 move was inaccurate.
! This is the key move that holds white's position. Black cannot contest the long diagonal with a Q vs a B.
17... f4 18. f3 fxe3 19. dxe5 Nd7xe5 20. Ra1e1
The pinned N gives white a chance to recover the e pawn and defend f3.