What if an observers has not uploaded the DAR?

In the following template…

template Data with
    publisher : Party
    subscriber : Party
  where
    signatory publisher
    observer subscriber
    
    choice Data_Acknowledge : ()
      controller subscriber
      do pure ()

…can the publisher successfully create a Data contract if the subscriber party has not yet uploaded the DAR?

If the contract cannot be created, then how is that different than creating the contract when the subscriber’s node is offline?

No, if an observer has not uploaded and vetted the DAR, then the transaction cannot be created.

This is different than the offline observer scenario. In that case, the topology information (which lists parties and vetted DARs) is still available to the submitter because that information is retrieved from the synchronizer.