Was there ever a plan for all 700s to be 12 cars?
Yes there was initially.
No, there wasn’t.
The reason Mill Hill, Elstree and Radlett have 12 coach platforms is because they were going to *some* 12 car services, nominally from somewhere on the South Central. But there was never a plan for Hendon, Cricklewood or Kentish Town to be served by anything other than 8s.