Azure SQL Database patch and update doubts
Is it possible for the customer to reverse a patch, or test it and authorize it before being applied?


No.  It’s part of the service definition that updates won’t break your application.  Any potentially breaking changes, including query optimizer enhancements won’t be active until you opt in by changing your Database Compatibility Level.  

See 
https://docs.microsoft./en-us/sql/t-sql/statements/alter-database-transact-sql-compatibility-level?view=sql-server-2017