Of course UA interlines with other carriers. During irops you might get lucky to be put OAL, but this is YMMV even for 1K/GS depending on who you talk to. For a group of 'no status flyers' flying out of ultra small airports you are very unlikely going to find a situation where UA would find an option to re-route you OAL. There is certainly no obligation for an individual traveler, let alone a group.