Re 2 - Blackberry Messenger used an eight character alphanumeric ID, and was enormously popular in the UK (not sure about other countries however) - until Blackberries died a death at the hands of Apple and Android...
The problem was when you change your device you can't migrate your PIN so I think it contributed to decline to usage as people moved to other BB devices or iOS/Android
This only strengths the argument why majority of chat apps use phone number as your username.
https://www.theguardian.com/media/2011/aug/08/london-riots-f...
from 2010 / 2011
> Using BlackBerry handsets – the smartphone of choice for the majority (37%) of British teens, according to last week's Ofcom study – BBM allows users to send one-to-many messages to their network of contacts, who are connected by "BBM PINs". For many teens armed with a BlackBerry, BBM has replaced text messaging because it is free, instant and more part of a much larger community than regular SMS.