Yes, +Spell Dam does affect DoTs, only it's through Coefficients. This is just a very simplified example, look on wowwiki.com for exact numbers. Say you have +100 Spell Dam. Corr will receive, let's say 25%. So you get 25 extra damage over the course of the spell, so like 5 extra each tick. Look at Wowwiki for exact numbers, mine are pulled out of my ***.
SL/SL is refering to a talet build that goes far enough in Affliction for Siphon Life and far enough in Demo for Soul Link.
SL/Ruin is enough in Demo to get to Soul Link and enough in Destro for Ruin.
Any other questions feel free to ask, we'll try to help.