He's not a "bit" out of the GOP mainstream; he's completely outside the GOP mainstream on conscription, on this incest thing, on the 17th amendment, and on slavery reparations.
I would vote for him if I were in Illinois, but the man would never win a GOP primary with those views at any other time.
The incest thing is not an 'issue'...it is simply a lesson in logical thinking for liberals, given in response to a question.
I would guess that a majority of Republicans would heartily support the repeal of the 17th, and the 16th Amendments.