No. It is mandatory that you have to use Jio sim card in JioPhone.
Yes, JioPhone does have a 3.5 mm headphone jack.
Yes, JioPhone comes with inbuilt games.
Yes, you can type message in the Gujarati language in the JioPhone.
Yes, you can use Airtel sim card in the JioPhone but the Airtel sim card will work only on 2G network.