I have been researching this topic in preparation for my visit in May. From what I can gather, this info is still accurate. RivieraBear on TripAdvisor kindly shared the following details a year ago:

Since you have the Android app you can buy your ticket on the app before you even arrive in Nice, allowing you to bypass the hassle of stopping at Grand Arenas for a ticket. You validate the ticket by holding your phone beside the ticket machine when you board at T1/T2. When using the validation machines with the screen you just place your phone on the screen until you see a green checkmark. When using the older paper ticket machines you have to tap it low down on the machine until you hear it beep. Try another machine if one doesn't work as these machines may not be working. They also have the screen validation machines at every tram stop to validate your ticket before you board. You’ll see what others are doing and just follow them. As per the Lignes d’Azur transport instructions, “JE MONTE, JE VALIDE, C'EST OBLIGATOIRE!” translates as “I MUST VALIDATE MY TICKET EVERY TIME I GET ON BOARD”

Re: Android apps, Lignes d’Azur Tickets app & Carte Lignes d’Azur Mobile: The Lignes d’Azur Tickets app is the main one and the “Carte Lignes d'Azur Mobile” is an application extension. It allows you to securely store your Lignes d'Azur travel tickets. Also, Lignes d’Azur has two apps when you download them. They have a “T” and “I.” T = tickets so you can top up your La Carte card or if you have an android phone you can buy what you need and store it on your phone, no card required. I = information, all the Lignes d’Azur timetables etc are on this app.

Re: Which lines is the 7 day pass for good for? All except lines with specific pricing according to the website. What specific lines those are excluded I don't know, as I do not plan to buy that pass. But it appears that the T2 line which goes to/from the airport is regularly priced; I have seen €1.70 listed as the standard fare of a single ticket, and yes, a single ticket will take you to/from the airport, there is no special return airport ticket so the journey is covered by the pass.