As far as Carrion Swarm? I just have Meta tied to an easy hotkey, and since it doesn't trigger the GCD I can just hit Meta then Swarm. What you might want is:
#showtooltip Carrion Swarm
/cast [mod:shift] !Metamorphosis
/cast Carrion Swarm
which will cast Carrion Swarm in Meta/DA, or if you're in Caster form and holding Shift, put you into Meta just long enough to Carrion Swarm, and pop out immediately. It will trigger the Stance cooldown though.
edit: the last line might need to be
/cancelaura [mod:shift] Metamorphosis