If one entity in the supply chain uses the disaggregated default value (DDV) for a component of the GHG formula, are all other entities in the supply chain also limited to DDV for that element?

Yes, this is generally the case. A switch to actual values is only possible if complete information regarding the relevant formula element is approved and available from all prior entities in the supply chain.  For example, if DDV is used by an intermediate processing unit for the GHG component ep, then the final processing unit must also use DDV for the GHG component ep.