As getValue() returns null throwing an exception in isReadable() seems inconsistent to me. Though changing getValue() would be a BC break.

The method that would throw an exception is readIndex() that is called for both isReadable and getValue, such as readProperty for an object that behaves as proposed here.
Maybe catch the exception in getValue to keep BC, WDYT? That would fix isReadable at least.

Actually, isReadable() always returns true for an array because readIndex() doesn't throw an exception if the index doesn't exist and isReadable() returns false only if an exception is thrown in readIndex()/readProperty().

So with this the return type would stay intact, just fixed as it is bugged.
In addition of throwing a NoSuchIndexException in readIndex() to return the good boolean, I propose to catch the NoSuchIndexException in getValue() if throwing it would break our BC promise (not sure that don't catch it would break it as the thrown exception should be already expected).