Using

record.copy

, is it possible to edit the subsidiary on the resulting pre-filled form?

record.copy returns the record.Record object. So, I don't see why you cannot edit the pre-filled fields.

if its in dynamic mode and you change the sub I think that might cause issue with other fields... dynamic mode cares about order of operations on when things get set, so if you set something that needs to be set before something else, it will null out other fields

Just needed to set the sub before the customer.