Namaskaar Vivekji,
Since I have no exposure to practical analysis of charts, I will have to answer this based on whatever I have read (which is not much) and whatever I have understood (which is of-course even lesser

). Hope you will correct me.
I feel placement of Shani in dusthana in general can also lead to some good results (containment of bad effects), though the evil effects are not fully nullfied.
As for kaaraka destroying the significance of house if placed there, I keep reading affirmations and exceptions. I again get a mixed feel (not all effects are destroyed...).
As for comparison between placement of Shani in 6/8/12 in Vrishabh/Tula as against Kumbh/Mithuna/Kanya, I feel it should prove more auspicious for the latter since in the former case, as a single handed yogakaaraka it gets locked up in dusthana and in addition does not own any dusthana.
CRS