Players will be allowed to roll any dice out of combat. PHB rule
DM make an ability check when a character or monster attempts an action that has a chance of failure and narrate result accordingly.
Thais will include initiative rolls. Once initiative is rolled we will switch to a different thread where combat will take place as usual for GP - players will be doing all the attack and damage rolls on their turn.
Given amount and the influence of reaction abilities a LVL 15 PCs could have I will see to add reaction windows to the combat game peace and other means to allow players interrupt combat at any moment.