We started in the afternoon from MB and first slept in Ruška koča on Areh, second day in Ribniška koča. Definitely recommend advance booking, not for crowds but so they know you're coming. In Ruška koča we were alone and if we hadn't said we're coming, probably arrive at 20:00 to closed hut.
Last day descended to Slovenj Gradec, bus to Dravograd then train to Maribor.
Mariborska koča definitely closed Mondays.