1) Doug wants to be mayor. He's hated every second of playing second fiddle to Rob.
2) Doug will do debates. Any other human being would have stopped going in front of the cameras if they were as stupid as Doug, but he does multiple scrums a day. He won't stop because he can't stop.
3) As much as I am completely willing to believe that the Ford's are milking Rob's illness, I don't know what gets accomplished by pursuing this angle. Think it through: Unless we can find a doctor or a member of Ford's inner circle to admit that, say, Rob had a colonoscopy done at GreeneStone in May and discovered a "mass" that he already knows is benign and put off surgery until he realized he couldn't win (a not implausible scenario), all we'll have is a bunch of circumstantial evidence. Any questions directed toward Doug on this (because we know that Rob and Mikey won't be doing any interviews) will be met with "Tumour. Tumour. Tumour. Tumour..." Which brings me to
4) To be honest, I don't know how one campaigns against Doug because it's quite obvious that being a serial liar with utter contempt for rules and basic human decency has been no barrier to him getting this far. If he's managed to stop people from coming forward about his hash dealing days (and whatever else) so far, is anyone likely to come forward now?
I'm not saying that he'll win because if it looked like that might happen there would be a huge amount of pressure on Tory or Chow (but, let's be honest, Chow) to step aside. But I am saying that I don't know what will happen to change the dynamic of the current situation where he can lie and be an asshole with impunity.