Yes... the so-called sketches were often done after the major work was already completed. Kind of like reverse preparatory work. His work was very expensive indeed, and tthis was one of the ways they funded it. That and tax write-offs from big buisness.

I never understood the Jean Claude thing. She always insisted very vocally that she and Christo were a creative partnership. However, there is no discernable difference betweeen Christo's work before JC, and that which he produced after they partnered. So where is her creative inmput?

My experaince of her is colored by a very colourfull fax I recived from her whilst interning at a major London gallery. I made the huge mistake of addressing a communication only to Christo.

No, it’s not that common. While most artists stop at sketches - and Christo sketches are enough to grant him a relevant place in modern art (it really worths to study them) - he always pushed to realization whenever possible.