(Not a teacher, but a Scotsman)
Q1: Yes, the journalist is taking 'creme de la creme' which I suppose means people who are of the highest social level, and is making it 'scum'. So he means people who are of the lowest social level. So yes, it means the worst, by social class. Also, I presume the writer didn't coin the phrase, and that it has been used before to describe Rab C Nesbitt, who would fit the description.
Q2: Govan is a place in Scotland, and it is generally viewed as being a place with mainly working-class residents. As I said, Rab C Nesbitt was set in Govan, and so when he said 'Govan's 'creme de la scum'' I assume he means the character Rab C Nesbitt.
Q3: The Incomprehensible Scotsman is Gordon Brown, I presume. Tilting at Windmills is an idiom which means 'to fight with imaginary opponents' - presumably referring to the climate change summit.
So, really, he means "If I want to see an incomprehensible Scotsman tilting at windmills, I would rather watch Rab C Nesbitt than the Prime Minister".
I hope that it is more clear to you now.