The entity and the account dimensions work in inverse of each other. When you think about entity you can "Share up" but in Account you Extend down/out to lower level dimensions. For example if I had your two of your three account dimensions and I wanted Corporate to inherit Capex then Capex would be the "parent dimension" to Corporate but Corporate would inherit all of the dimensions. Also you can only extend a member once in a dimension and this includes if I create a new member in Corporate in this example and add a relationship from a Capex member under that new Corporate parent this would count as extending the account. If I understand correctly what you are trying to do I don't believe you can do what you want easily leveraging extending dimensions. Let me know if you want to chat about this.
