when Sm and Sa are conducting the current charges L1.
When Sa and Sm stops conducting, according to faraday law L1 oppouses to abroupt changes in current so it uses energy stored in their magnetic field and acts like a current source, and Aacording to the previous current direction the new currents is in the same direction and D12 complets the path so current can happens.
You wrote: when Sa turn OFF, C2 discharging, current flows throught the anti parallel diode of Sm...... L1 & C1 form a loop together with D12. Not anti paralel diode of Sm, because VDC1 should be greater on the other side.
You wrote: when SM turn OFF, D12 again 4ward bias & form a loop with L1 & C1....
but remember that when Sm and Sa turn off voltage in the inductor changes it's polarity