Can a Canadian Citizen sue an American Citizen?

Yes. The only key is proving the agreement, its terms, and the breach and the damages. Good Luck.

Yes, but there are many, many variables. This is definitely a matter about which you must consult an attorney.

Yes. As the other attorneys said, there are many other factors that must be considered to determine whether a court would enforce the contract. It would be best to have an attorney review the terms of the contract to see what type of relief might be available to you. Best of luck.