Microsoft Bookings behind the scenes

I'm going to caveat my answer and say I am not a total expert and don't work for MS! 🙂

• 
  Who is the Organizer? Is it the new Exchange account that is created or the booked Staff user Organizer or Presenter? What if the booking is re-assigned?
  Yes, it is effectively the calendar account is sending out the invites to the meetings when someone book a slot on a service. However the staff member shows as the organiser in the actual meeting (assuming you are using this for teams). 
• What if the booking is re-assigned? Not sure what you mean here? If the staff member cannot fulfil a service appointment, the Bookings administrator or Bookings scheduler can assign another staff member, as long as they are associated with that specific service. 
• Can the Meeting Options be accessed prior to the meeting? What are the default settings for the meeting? The booked staff member "could" change the meeting options in advance but only by going into the actual meeting and doing this. They don't get the "meeting option" in their booking. The default settings are whatever your o365 team have set in admin, basically follows what your usual teams does. 
• Is the Customer an Attendee by default for the meeting? Again depends on your tenancy settings. For me, anyone booking in my tenancy joins essentially as a presenter, but if an external joins they are an attendee. The  booked staff member can control everything in the meeting though.
  
• If I add additional Staff to a booking, are they Presenters or Attendees? (since we cannot book a group of Staff members in a single booking automatically, we are having the Staff person edit the booking to add the additional Staff assigned to the Service so they get the meeting invite).  See last answer, I'm afraid you will have to play around and see what your tenancy is set as. You can share the meeting link with them, but only the staff member and the customer get the link.