A good justification for (b) can be found by determining the genetic diversity of indigenous populations from thousands of years ago and comparing to the present day. As an example the UK has had many 'immigrants' in just the past two thousand years alone, with the Romans, Saxons, Vikings and Normans being notable topics on a good school's History lessons.
Nevertheless, I believe your arguments aren't particularly relevant to the issues held by these extremists, who fear Islamic culture more than they fear Muslims themselves. They view 'multiculturalism' as the replacement of indigenous culture. If you want to go anywhere with this, you have to debate this more nuanced (and less scientific) viewpoint.