You are right "Herkese Merhaba" means Hello everyone. Turkish people generally use "Merhaba" or "Selam" when they see their friends. If you want to talk with a group (more than one person) or if you are in the meeting, you can use "Herkese Merhaba".

Yes, you can use Herkese Merhaba if there are more than 1 person

Yes it is correct to say ‘Herkese Merhaba’ . Also merhaba’ you can use for informal and formal situation :)

😍

Herkese merhaba