1. A Miasma 2 with contagion is 120 potency and so is Ruin 3. I'd stick with ruin 3.
2. This is in Hai Hai's guide on pg 30. Long story short don't use shadowflare in DWT for single target.
3. Out side of the opener on single target, you want 2 festers during a DWT. Sleigh's opener is based on getting the most use out of your pot and group wide damage buffs.