Found this on Southwest website - missed it on search before my original post -

"If I’m traveling on a multiple-passenger itinerary, can I checkin via the mobile site and app?"

"Yes, however, the mobile checkin process will check in all Passengers included on the reservation. Unlike online checkin at southwest.com, there is not an option to check in one Passenger at a time. If you wish to check in a single Passenger traveling in a multiple-passenger reservation, please do so at southwest.com. Multiple passenger itineraries are not eligible for mobile boarding passes at this time."


So, this seems to answer one question but leads to another -

If the mobile app will check in all pax on the reservation that's perfect - but it appears we will still need to go to Southwest.com (or kiosk at airport) to print boarding passes if multiple pax itineraries are not eligible for mobile boarding passes ?