Yes, you can use Airtel sim card in the JioPhone but the Airtel sim card will work only on 2G network.
Yes, JioPhone does have speed dial functionality.
JioPhone does a plastic keypad.
JioPhones comes with 4GB Internal Memory - so just like a normal Android smartphone, you can save as many contacts depending on the free space in your phone. In practial sense, you can store unlimited contacts.