[RPG] Can Kineticists accept zero Burn

Sacred Weapon stacks with other weapon bonuses. A warpriest simply cannot raise the enhancement bonus above +5, regardless of the sources of that enhancement, so if a weapon has some enhancement already before using Sacred Weapon, normally some bonuses would be lost. However, instead of increasing the enhancement bonus further, you can bestow special abilities upon the weapon, that replace some of the enhacement bonus you might have had, including the amounts that would go over the maximum allowed +5.

1. Precisely, but you don't have to spend them on abilities. You can simply lose them if you prefer. Why would you like that is anyone's guess.

2. No. You can only replace bonus that would be the result of Sacred Weapon. You cannot replace an enhancement bonus that has some other source. While this is not explicitly stated in RAW, there is a passage saying

   The enhancement bonus and special abilities are determined the first time the ability is used each day, and cannot be changed until the next day.

   That means you choose your enhancement bonus and special abilities without regard to existing bonuses and abilities on the target weapon. You might even choose ones that you have from some other source (spells, weapon enchantments, other player's abilities) and in that situation the enhancement bonus will stack but not exceed +5 in total and the doubled special abilities will not stack. If you decide to use the ability on a different weapon, you have to use the same choice of bonuses and abilities and resolve the final effect independently

3. This fragment means that when Sacred Weapon ability takes effect the enhancement bonus is considered first. In a situation where the weapon is left with no net enhancement bonus, no further special abilities will be added.

No, because of the action economy

Kinetic Blade and Energize Weapon are both form infusions which must be used as part of an attack action, charge action, or full attack action, which means they cannot normally be used together; a kineticist can only use one form infusion and/or one substance infusion on a single invocation of kinetic blast:

Each time the kineticist uses one of her kinetic blast wild talents, she can apply up to one associated form infusion and up to one associated substance infusion.

With the normal action economy you could only possibly make one of these actions on a given turn so that precludes you being able to invoke both infusions at once, and the kinetic blade only lasts until the end of your turn, so it's gone before you can use it in another attack/charge/full-round attack action.

What if I have extra actions somehow?

If you were to have access to extra standard actions somehow (such as granted by certain Mythic effects, for instance) you could conceivably be able to create the kinetic blade with a standard attack action and then make another attack action with it, applying Energize Weapon with the second action. This does require you to consider the kinetic blade as a "manufactured weapon", but the precise definition of that is contested. There doesn't seem to be specific clarification from Paizo on whether or not the kinetic blade is a manufactured weapon or not, but the similar case of the Warlock's mystic bolt was addressed in FAQ:

Warlock Mystic Bolts: This ability says that it lets me count it as a light weapon, but is it manufactured? Do the bolts add my Strength bonus on damage rolls?

Warlock mystic bolts aren’t manufactured weapons; they’re magical effects (similar to scorching rays or the flames from produce flame), and you don’t add your Strength bonus on damage rolls.

It looks like maybe a kinetic blade shouldn't be considered to be a manufactured weapon, as it is very similar to mystic bolt... but the whole manufactured/not-manufactured distinction is pretty muddy and awful. It seems like it was sometimes used as a catch-all to mean any non-natural weapon, but in others has a definition a little more in-line with standard English. In order for the system to make any sense, some common sense reasoning generally needs to be applied to determine which meaning is meant in context and whether or not an effect is applicable.

Personally, I'd rule that it's okay to combine the two effects if you've got the actions necessary to make it happen; there are worse things you could be doing with extra actions, and it would let you combine different blast types, which is cool.