coming to europe in May need help with seat reservations and itenery

1. You can put all passes one one phone, or on several phones. You can't travel separately if you put them all on one phone. Internet is only needed to activate the pass (once), to activate a travel day (by adding the first train of the day, after that you don't need internet to show the ticket or add/remove trains), and once every 72 hours.
2. Yes you will, since a reservation is tied to a specific train. Your example can be done entirely reservation-free though. In Switzerland in general, you don't need reservations, not even for 5 people.
3. If the seats are full, they're full. There may be fold-down seats if you're lucky.

I'll add though that a full train is rare in Switzerland (only on suburban lines or after a disruption of some kind). Even at rush hour I have a seat. :)

For example in Zurich simply show up 10 min before departure and you'll have plenty of seats to choose from. The front carriages are less busy.

Nobody books seat reservations for domestic journeys in Switzerland, I live here and never made one. The network works like a giant subway system. ;)

The Zurich to Como direct trains require a 13€ reservation for barely a few km in Italy -> not good value. Instead take any train to Lugano or Chiasso and change there for Como S. Giovanni (plenty of suburban, regional trains).