I don't have a family account so cannot comment on that.
You can add as many phones/tablets/pcs as you want when you log in with the same amazon account all can control the same echo's.
You are correct when saying some features are USA only. Depending on where you are, starting that amazon account from the USA might be worthwhile. However, if you don't use any of those features, are in a country that had pretty good alexa support for local stuff or need a specific skill that is only available in another country, make sure you make the right choice which country you start off in.
In my specific case, logitech harmony is a USA skill not available in The Netherlands and alexa support here is still meh so I prefer a USA account, or UK that works too.