I have CoreData app that is perfectly working on iOS10, written in Swift3, supporting iOS 8.4 and above.
When I try to run it on iOS 9.3.5 I'm getting error:
2016-10-07 17:47:20.596 FormApp[710:179733] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSSet intersectsSet:]: set argument is not an NSSet'
CoreData: warning: Unable to load class named 'FormApp.Form' for entity 'Form'. Class not found, using default NSManagedObject instead.
let form = NSEntityDescription.insertNewObject(forEntityName: "Form", into: managedObjectContext) as! Form
For some reason NSSet is expected, but your NSManagedObject code has NSOrderedSet, which is a subclass of NSObject. Try to remove "Arrangment: Ordered" checkmark in your core data model and refactor those relationships to NSSet. Not sure why this happens in iOS 10 but not in iOS 9 though.
P.S. Perhaps you should reconsider your Core Data model? It looks like your Open/Closed questions are going to change their status. If so, I would recommend to make one Question entity with
closed bool or