Avatar of Fire question

Hmm, so Avatar of Fire converts 50% physical/cold/lightning to fire, and deal fire only... ok... so if I used lets say Sunder with Avatar of Fire, then 50% of my physical damage would become fire, then the other 50% physical damage just... DISAPEAR!?

Yeah. Poof - gone.

That is in fact what happens. You would need to find a way to convert the other half of your physical damage to fire damage. There's a few ways to do that, but much like every other proper Keystone in the game (so not Vaal Pact), you generally need a reason to do it.

Avatar of Fire is one of those that looks incredibly awful and 100% never worth it until you get a really good grasp on how a specific game mechanic works - in this case, conversion. AoF exists to allow you to get skills fully converted to Fire that otherwise would not be able to be fully converted to Fire. For example, Sunder:

Sunder: 100% Physical

Sunder + AoF: 50% Fire, 50% Physical. Deals 0% Physical damage

Sunder + AoF + Hrimburn: 50% Fire, 50% Cold. 50% of 50% Cold is converted to Fire by AoF > 75% Fire, 25% Cold. Deals 0% of Cold damage

Sunder + AoF + Hrimburn + Cold to Fire Support: 75% Fire, 25% Cold. Remaining 50% of original Cold damage is converted to Fire. You now deal 100% of original Sunder Physical damage as Fire damage.

This seems pointless - until you realize you've now also converted 100% of your Hatred damage to Fire damage, allowing it to stack under a single Fire Pen jewel, and that you can also now use Melee Physical Damage and Elemental Damage with Attacks together. Even then, it's more a trick build than anything else - but using AoF to finish conversion on existing 50% Fire skills is also very useful, and using it to change the damage types of other skills to interact with different items or Ascendancies can also be useful.

'Deal no non-Fire Damage' is honestly not a huge drawback given some of the crazy things you can end up doing with Avatar of Fire.