Damage Conversion and Penetration Question

Alright so I've read the wiki about how damage is converted if you are doing 50% lightning 25% fire and 25% cold that each one would be 50/25/25 if based off 100 damage.

My question is when does penetration come into play? Is pen calculated beofre or after the conversions have been made?

I'm doing a Whispering Ice build with Cold to Fire and Avatar of Fire, 100% cold - fire conversion and then using emberwake for ignite stacking. If a friend is cursing with frostbite, is that useless or does it calculate the cold damage with the pen provided by frostbite then converted?

Additionally, if ele weakness is on the mob does that effect the initial cold hit and then the conversion to fire damage? (seems unlikely to be able to double dip)

Once the calculation of your damage output is done conversion doesn't come into play anymore. After your damage output is determined the game calculates how much damage you will actually do to an enemy, which is where resists and penetrations come into play, for that part of the process using 100% converted ice storm is exactly the same as using firestorm.