Comio S1 does have a very decent battery back up. It has 2700 mAh Li-Polymer non-removable battery which offers almost 15 hours of talk time and up to 143 hours of stand by period.
Yes, Comio S1 does have support for Jio VoLTE also. You will be able to use Airtel VoLTE as well.
Yes, Comio S1 does have the VoLTE support for Airtel.
The standard packaging of Comio S1 includes - Mobile phone (with battery), USB Cable, USB Power Adapter, Ear Phone, User Manual, Warranty Card, Sim Ejector Tool, Screen Protection Film & a Free Back Cover.