Possible Clues:

• Debater for the opposition, at times
• One arguing for the unpopular side
• Role played to provoke a debate
• One deliberately introducing a controversial idea to debate
• One taking opposing line to provoke debate
• Provocative debater makes David eat cloves
• Diabolical barman who attacks saintly character?
• Contrarian to go astray, drinking Calvados, wanting a cocktail
• Argumentative type who's opposed to enlargement of calendar?
• One who argues for the sake of arguing