3) What happens if I book to travel after 29 March 2019 and my holiday cannot go ahead due to Brexit?

There is nothing to suggest that you will not be able to continue with your holiday plans after 29 March.  Even in a no-deal scenario, the European Commission and UK Government have said flights to and from the UK will still be able to operate.
Customers who book a package holiday with a UK travel company enjoy the most comprehensive consumer protection:  if you book a package, your holiday will be protected under the Package Travel Regulations, meaning you have a right to a full refund if your holiday can no longer be provided.