I did make the unconcsious assumption that this was on top of technical ability.

There again, there is a natural tendency to judge a persons technical ability on the basis of their personality. Certainly people tend to denigrate the talents of those they don't like, or see people who disagree with them as fools. You'll very rarely get the 'real deal' in an interview. So after you've cut out the total non starters, just just need a suitable framework for filtering the rest of the candidates, becuase from then on 'you are on suck it and see'.

As for the last point, if you want to use it you should tell the candidate, otherwise you will get people not arguing with you because they are following rule 1 , tell them what they want to hear.