I would assume as a new airframe, there was an initial group of pilots that were trained and flying as two captains paired together.

When did Envoy start training first officers for the 175? This should give an idea of when those pilots are eligible for an upgrade to captain. If Envoy does not continue to receive more airframes, pushing guys out the top of the FO stack, then upgrade of current FOs is the only way for movement at the bottom of the reserve list.