This is rather difficult to test. CoreLib is not roundtrippable through ILDASM/ILASM (error : Invalid Get method of property 'FirstChar') so I can't really add a new default interface method to, say, ICollection<T>, and test it.
We should have a look at this when we pick up a version of Roslyn that supports default interface methods.
Yes, but since this only affects interfaces that are 100% under our control, we can fix this if/when this becomes an issue (i.e. the libraries team decides it wants to add new methods to e.g. ICollection<T>).