So the App requires you to have an Amazon account to use it which in turns links to the Alexa. I don’t think it needs to be a paid for Prime account as my son has his linked to his own Amazon account and he doesn’t have Prime.
You can have several Echos on the same Amazon account and they will appear in the app. When I set my parents one up I used my dad’s Amazon account and put the app on his phone (he doesn’t need to do anything with it). This was to enable the “call” function. I didn’t want their Echo on my Amazon account as they would then get all my notifications which would have bothered them.
I agree with the other poster in terms of where to locate the screen. Remeber that if you put the screen in the kitchen, when they’re in the living room they may not see it.
My plan with my parents had been to have three. Living room, landing, and garage (which is loosely my dad’s workshop). My reasoning was, if they fell they could call from almost anywhere and one of the Alexa’s would “hear”.