u could potentially look into this entity, i'm not sure it will help but it might, u can detect the entity and check the flags to see if its the finale nav...u can clear the attributes or mess around with it and see what u can break. Trial and error is your best friend
https://developer.valvesoftware.com/...tribute_region Other than that i'd recommend doing distance checking between the survivors or specifically on that map just teleport everyone to the radio button when you've pressed it, although doing that would teleport survivors from anywhere on the map to the finale area. You could do a 5 minute cooldown which gives everyone 5 min or whatever to get rdy before u do that as well, to prevent premature teleportation if someone rushes to the radio before everyone else. There are options just would take some time to code and get right. This entity may help too
https://developer.valvesoftware.com/wiki/Trigger_finale use it to detect the button push maybe.