Hey.
First off I simply can't figure out if the Kinetic Blade is just a normal melee attack or if it's meant to be a melee touch attack. Furthermore the Annihilator archetype changes it up more, is that one the same in this case?
Secondly, is the kinetic blade (and annihilator variation) useable with Weapon Finesse?
If it's an energy kinetic blast (fire, cold, lightning, and few of the composites) it'll be a ranged touch attack, and a melee touch attack with kinetic blade or whip.
The annihilator wants to start off with a physical blast as its devastating infusion at level one as it only works with physical blasts (which attacks regular AC), so when it's ever used in melee it's a regular melee attack.
If you make the kinetic blade a light weapon, you can use Weapon Finesse. Nothing states a devastating infusion needs to be one-handed size for 1.5 x Con if held in two hands, so make a light weapon size and hold in two hands should be able to use Weapon Finesse and do 1d8 + 1.5xCon.
So in essence I could make it work like Elven Curve Blade and I don't need to be proficient with it?
Kinetic Blade's and Devestating infusions shapes are entirely cosmetic and have no effect on any mechanics. Devestating Infusion lets you "2-hand" your blast, but that has nothing to do with what shape you choose for your weapon.
However, melee touch attacks are finessable because they are considered to be using either you unarmed strike or any available natural attacks, which are all light weapons.
That's not entirely applicable to the OP's question. If the kinetic blade is a physical blast, and the devastating infusion has to be a physical blast, then "melee touch attacks are finessable" aren't in play. It is dependent on the size you pick for your kinetic blade. Light weapon, can use finesse and Piranha Strike. One handed weapon, no finesse and Power Attack. Devastating infusion with light weapon in two hands, can finesse 1d8 + 1.5 x Con damage and can Power Attack for -1 attack +3 damage and attack regular AC.
Kinetic Telekinetic blade would be entirely dependent on size of weapon on hand.
The shape is purely cosmetic, but the size isn't. There's nothing written in the kinetic blade entry that allows Weapon Finesse to apply to physical kinetic blades except for "you create a non-reach, light, or one-handed weapon". If that line wasn't there, physical kinetic blades wouldn't be finessable at all. Even if it's not wielded, there isn't anything that supports an "armed unarmed attack" either; Weapon Focus (unarmed strikes) wouldn't work with kinetic blade (instead that'd only work with kinetic fist), and Weapon Focus (kinetic blast) is used instead.
Found Mark Seifter's playtest post on Weapon Finesse with kinetic blade
So if it's light regular kinetic blade, no holding it two hands and expecting any benefit (like say with two handed Power Attack) but it'll be fine if it's one-handed.
For devastating infusion I think it's a specific case where you can still hold two handed even if light for 1.5 x Con but I'm not sure about two-handed Power Attack bonuses anymore.