As health-insurance cooperatives created by “Obamacare” collapsed like dominoes in the mid-2010s, the Montana co-op faced its own multimillion-dollar chasm, created by an obscure political deal in Washington, D.C. But the Helena-based Mountain Health Co-op scratched out a $15 million loan to stay afloat — and this month hit its highest customer numbers ever, as one of just three Obamacare-created co-ops still alive nationwide.