Before Save Session Registration Validations Flow replaced with Before Insert and Update Session Registration and canceled Session Registrations excluded from overlap check.
After Save and Update Session Registration Flow created and managed to count Session Registrations under related Event and updated the related Event Registration.
After Insert Update Event Registration Flow updated and managed to cancel the Event Registration when there is no active Session Registration related to it.
Duplicate and Matching rules created to prevent duplicate active Session Registrations while excluding canceled ones and Canceled_Text field were created to use Canceled checkbox value at matching criteria.
Number of Registered Sessions field created on Event Registration for to count related Session Registrations.
Rollup field updated to exclude canceled Session Registrations.
Before Delete Session Registration Flow created for updating the "Number of Registered Sessions" field if a Session Registration got deleted.
Test Cases
Org Credentials