Shadow masters resists Do NOT max at 75. This is not due to gear spawn either. Please be careful when responding if you are not sure of your facts. AT lvl 20 master has 75 %, at lvl30 82%, lvl40 85%, lvl 50 88%, and at lvl 55 89%. Not that you would get a S/master that high a level but even at lvl 21 they exceed 75%.
CoS doesn't work on ancients or any boss for that matter. The shadow master never casts CoS either ( you might want to confirm this but I have never ever seen it ). I recall doing ancients with high lvl SM on my untwinked phoenix striker and there wasn't any problem. The SW will use CoS of course.
The shadow master does cast cloak of shadows. I did not mean the -enemy defence effects but the bug that causes the game to sometimes crash when casting cloak of shadows on the summit, or at least in some way related to the skill being used on the summit. Encouraging to hear about you being able to fight the ancient with a shadow master, though.