1. Yes that's possible.
2. That depends on on how many phones the passes will be. Theoretically you could have all passes on one phone, but you then always have to travel together. The best though is that each person has their own pass on their own phone, so everybody can travel independently if needed.
3. Not really. The person who buys the passes just needs to make sure they have all the correct names and birth dates.

Thanks very much for prompt reply.

If one buys the passes for all 6 people initially, would it be possible to save a copy of the passes by other 5 people in their phone too, just for reference and convenience and as Plan B just in unforetunate circumstances that the main purchaser loses its phone.   However, we all 6 will definitely travel together. 

A mobile pass can only be on one device. You can move it to another device though if something happens to the device on which it is.