Jar, have you ever read "Guns, Germs, & Steel" by Jared Diamond. His book answers your third question about draught animals. It appears you can only domesticate animals that have the ability to be domesticated. The zebra is one animal that comes to mind as they are so similar to horses but are just too mean and stubborn to do what you want then to do.
As far as the wheel in the Americas, it appears that some indians used them on toys and that in South America they may even have had potter's wheels. Link:
http://www.trends.ca/~yuku/tran/xwh.htm