No, you cannot use Internet Calling Plans while connecting to a public Wi-Fi.
Yes, if the Internet Calling Plan is active on the eLife account, then all users of the supported apps connected to that Wi-Fi can make voice and video calls.
Some Android devices have an option to restrict notifications or background activities of apps due to battery optimisation features. Please make sure your battery optimisation function is off if your device has such feature. Also, please make sure you enable floating and lock screen notifications.
Yes, Just choose "Show iPad & iPhone Apps" in the iPad App store.
Yes, it will work on all SIM cards.