Unfortunatelly the implementation bugous as of now. I wrote additional tests.

self.parser = ErrorRaisingArgumentParser()
self.parser.add_argument('-a', action='store_true')
self.parser.add_argument('-b')
self.parser.add_argument('rem', nargs=argparse.REMAINDER)
self.assertEquals(self.parser.parse_args('-b 4 -a  -b 5'.split()), NS(a=True, b='5', rem=[]))

This part is OK. But with the new option:
self.parser.disable_interspersed_args()
self.assertEquals(self.parser.parse_args('-b 4 -a -b 5'.split()), NS(a=False, b='4',  rem=['-a', '-b', '5']))

This assertation also passes because it contains the actual result, which is unexpected. This is because the code doesn't handle properly the arguments that are non-options, such as '-b' in this case.

I can't see a good solution for this.