You can only enroll in one Challenge at a time, and cannot switch midway. And you can only do a given Challenge once. As noted, you don't need an AA flight for the Challenge, only to requalify in subsequent years for elite status. And you must be a Canadian AAdvantage member to be eligible for the CX credits on all fares [except the Asia Pass]. AA has very restrictive CX credit criteria. And segments don't count either for non-eligible CX flights.