This a two part question.

Is a proximity mine considered an obstacle after it has been deployed? And can a seismic charge attach to a proximity mine that has not been blown up?

proximity mines are devices and objects, but not obstacles. you cannot attach a seismic charge to anything currently. you can, how ever, place devices overlapping each other or other objects. proximity mines and seismic charges do no have any effect on each other what so ever, though, even if they overlap.