GaryT

In relation to using the Cdim7 as a substitute for D7 and B7, you should note that the dim7chors is the same as a dominant 7th chord with a flattened 9th (7b9 chord) with the root omitted.

  So Cdim7 = C Eb Gb A
  
  and:
  
  B7b9   = B D# F# A C     or B  [Eb Gb A C]
  D7b9   = D F# A C Eb     or D  [Gb A C Eb]
  Ab7b9  = Ab C Eb Gb Bbb  or Ab [C Eb Gb A]
  F7b9   = F A C Eb Gb     or F  [A C Eb Gb]
  
  In each case you have the Cdim7 chord over the root of the 7b9 chord. Leaving out the root leaves you the Cdim7 chord, which is why it can substitute for these dominant type chords.
  
  Mick