Yes - regardless of retreats during the regular combat phase, all Russian units can move and attack during the mech phase during the Soviet first Winter. We contemplated making this a separate special movement and combat phase after the mech phase during the Winter turn, but instead made it a part of the mech phase. Maybe making it its own special russian only movement and combat phase would remove some of the confusion.
Russian first Winter effects are only for territories in the original 39 Russian borders - so does not count in Eastern Poland or the Baltic States (or Bessarabia).