hi your diagnosis sounds possible to me. also make sure if there is some sound like crepitation. sometimes the peroneus longus tendon kind of luxates and "jumps" around in the cuboid area. also if peroneus longus is to tight, it can sublux. the cuboid bone. it often happens after inversion trauma. maybe also exercises for small foot muscles might be usefull to built up the arches of the foot and stabilize it. and muscular imbalances as a result of wrong/bad posture can make the foot instabile.
but i think you are on the right track!!
Cheers