Is it a specific requirement to "import" the private key into Jaxx? As mentioned with an xpub and a private key, the entire chain in that "account" could be compromised.

Would you not be better off simply sending those funds to Jaxx using the standard protocol?

Also, have you tried simply creating an unsigned transaction on the Electrum watching only wallet that includes the funds from the change address and then tried importing that unsigned transaction into the offline Electrum? This might trigger the offline wallet into "generating" the change address that you require.