First of all, I don't get why you would release the central tension with cxd4.  Qb6 is an option, but it catches up with you in the middle game.  According to the Silman Pawn Pointing rule, Black tries to counterattack on the Queenside.  Qb6 blocks the important b pawn.  The reason Qb6 is played up there is because all of White's pieces are stuck defending the d4 point.

In the advance, why stop attacking white's pawn chain with 5...Bd7?  Why not 5...cxd4 6. cxd4 Qb6 7. Be2 Nh6?

Occasionally, White will play moves other than d4.  He can play d3 and set up a King's Indian Structure.  Play d5 second move no matter what move he makes.  He can play Nf3, but it normally transposes into the exchange or the advance.

This is an accurate description of the French Defence.