Azure flame once activated requires two hands. While it is activated do you HAVE to use it every combat check? Can you switch to something else?

I would say you can. Why use it if you're up against a Magical Immunity creature? I mean, apart from the utterly bizarre "yay, I still get 2 hits for each 6 roll" crap (ditto for Shotgun vs Physical Immunity). The Azure Flame card itself only leaves if you refresh Call of the Azure Flame or somebody nicks that Spell from you.

Never liked that one, anyway. Every time I had to explain what is was to new player friends, I had trouble to stay objective, not mentionning I thought it was worthless. When an encounter asks me to look for the spell I want, I always take a tiny wither or a big shrivelling... can't help, nostalgic for very first original spells.