I noticed that you now have a section for boss encounters.
I don't believe you can FN off the legs to hit the boss, looking through some logs, none of the Enhancement Shamans are using Fire Nova. Personally, I never tried to use it during progression, and now on farm; the legs die to fast to waste time using a FN to test it.
In regards to talents, Ancestral Guidance is definitely the clear-cut winner. Because Ancestral Guidance does 40% of your damage, and attacking the legs gives you 100% bonus damage, thus the healing output of Ancestral Guidance will easily outweigh that of Healing Tide or Conductivity. There will always be times when a leg is up, and a crush is coming. In my guild, I Ancestral Guidance the 1
st / 5
th / 10
th Crushes, and there is always a leg available to attack, and I even have cooldowns up for every single one. 1
st Crush is Bloodlust / Fire Ele, 5
th Crush is Ascendance, 10
th Crush is Fire Ele / Potion. On top of this, I'm dropping a 5 Stack Healing Rain (Healing Storm glyph) on top of the main ranged camp, which also triggers Ancestral Guidance healing.