Yes, but if you've already got a proper phone why not use that with the SIM from Jio. Surely the whole point of JioPhone is for users that have no other phone as it is so affordable?
Yes, but the phone needs to be active and switched on for whatsapp web to work and AFIAK you can only have one whatsapp account per phone unless there is some weird hack I don't know about.
WhatsApp on the phone is effectively a server, and whatsapp web is a client. I can't see how their e2e encryption could work otherwise.
Could they be running WhatsApp, e.g., on an emulator? Then the jio phone is their only real mobile device. Maybe they’ve even found a no-cost cloud service to host the emulator?
Can't make heads or tails of this :)