5d MKIII Silent shutter mode

I cannot believe nor reproduce what you write! 1/500s is 1/500s at any drive setting. So the halo in you HDR must have a different reason: namely changes between the individual pics! Then it is clear that the longer the interval between the individual pics the more changes (by e.g. wind on flowers, leaves) can occur! But in all other uses single pictures have to be (by definition) unaffected by the drive setting. Even the contrary could be true: the silent shutter could cause less vibrations and therefore replace the mirror lockup . But this remains to be checked...