What I ended up doing is making a second button on a 'dead' button (one I don't have anything else on) that is like this;

/cast [nomod] Shadow Word:Pain
/cast [mod:shift] Devouring Plague
/cast [mod:alt] Vampiric Embrace

then I have my /castsequence for spamming. This is a tough one. I was having trouble wasting mana because I kept reapplying things on a long CD.