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Except for AA's distance-based "Explorer" awards, all AA awards are now one-way. On awards between AA's North America zone and another zone, a stopover at the North American gateway is permitted; it is not necessary to book roundtrip travel to get the stopover when using AA miles. The stopover can be of any duration, but all travel on each award must be completed within one year of award-ticket issuance.
As to whether a particular routing will be allowed when a partner carrier is used for the over-water segment, that depends on the partner carriers's published fare rules for cash tickets between origination and (final) destination.