In the US, "hey guys" or "hey everyone" are the most common. If you're in the south, everyone says "hey ya'll" (ya'll comes from adding 'you' and 'all')

Yes. Increasingly, US society wants neutered forms. Nowadays, actresses want to be known as actors! Chairperson is in more favor than chairman or chairwoman. However these does not apply to official titles such as Chairman Mao etc. Unfortunately, there is no such female leader yet...

All are ok!

All of those are commonly used in the UK, socially or at work.

Yes, you can :)