1. Don't interrupt ANY of your casts for a Proc (Fire or Thunder); you lose dps when you interrupt and you mess with your GCD as you previously stated. Complete the cast and THEN hit the proc.

2. Even if you are mid cast of Blizzard III you still do not interrupt the cast; you will once again lose dps and mana regen time. Continue to cast the Blizzard III for the mana regen, wait 2 ticks, and hit the Firestarter proc which will give you an instant cast of Fire III and give you 3 stacks of Umbral Fire. This is the way the mechanics work so you have less down time and more dps.