I'm not 100% sure I follow you, but:

Resist heat and resist fire are the same thing. Yes, they're useful to a felar.

Desensitize doesn't work exactly the way you think it does. Part of what you're missing, you'll notice if you pay attention to the affects command and it's relevant. (E.g., you won't see some affects or as much information about some affects while desensitize is going.)