Its random. They get a decision event where they can stay on the Axis or get a counter-coup and break from the Axis. If they do that, then you get a claim on Maribor as Germany and can launch the invasion. Once its all done, Italy will get the coast of Croatia, Hungary a serbian province and i believe Bulgaria gets part of North Macedonia
For it to work England must survive until 1943 or 44. They will get two events where they try to destroy the Heavy Water Plant (it doesnt really matter if they are Norway, RK or part of Germany) if they both fail, then you get the transport event and then a nuclear research site is built in edit: In the Tübingen province
I guess my issue is that once Yugoslavia joins the axis within a day the AI redeploys most of the units to the capital and most of the rest are moving wherever, and by the time the coup occurs the front with germany is wide open. the event should in someway lock all troops in place, until there is a coup or not. any coup that does occur happens within days of the signing. Not that they have a chance but it seems gamey that when the event to declare war happens the front is wide open and there is little or no combat until you reach the capitol.