Mastering the Elements: N. Jolly's guide to the Pathfinder Kineticist

[Tulya]

Notably, the Elemental Annihilator may be able to form a light weapon to benefit from Weapon Finesse, and also wield the weapon in two-hands for 1.5 x Con.
[Edit: Completely rewritten argument below.]

Devastating Infusion is merely a form infusion which modifies the function of your Kinetic Blast talent - you're still using the ability. This has some interesting rules implications because effects like Kinetic Blast don't calculate damage the same way most weapons do. For example, size changes should not alter the damage of a Devastating Blast any more than it does the Kinetic Blade talent it's explicitly based upon.

More pertinent to our fiendish purposes, the damage calculations wholly replace the calculations of your normal blast damage. As with Kinetic Blade, your Strength modifier is simply not factor in determining the damage, while your Constitution modifier is. This is in direct contrast similar effects where an explicit replacement/substitution occurs, implying inheritance of the original modifier's properties.

Thus, the general rule for Strength modifiers (a light weapon wielded in two hands does not gain the bonus damage) has no relevance to the specific rule (you get +1.5x Con modifier to your Devastating Infusion blasts if you wield them as a melee weapon in two hands) because the Constitution modifier to damage neither is, nor ever was, a Strength modifier.


*Nothing prevents you from either finessing a two-handed weapon (ex: Spiked Chain), nor wielding a light weapon in two hands. (Which is why there's a rule that you don't get the bonus damage when you do.)

**Kinetic Fist is not a counterexample to the above because it is nothing more than a damage buff based on blast damage dice applied to unarmed and natural attacks.