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 (i.e e_td)?

Yes, this is generally the case. A switch to actual is only possible if complete information regarding this formula element from all entities in the supply chain is available and verifiable. I.e. GHG emission for all transportation steps can be calculated.