The recoil from the exploding head provides a mechanism based on the evidence of what we can actually observe (matter driven out of the head at high speed).
I don't think you understand what the so-called "jet effect" entails.
Do you believe that the momentum of any matter traveling away from the head has an equal and opposite impulse on the head?
First of all, it is not about belief. This is demonstrated, repeatable, measurable, physical science: Newton's laws of motion.
If matter experiences a change in motion, there must be a force acting on it for a period of time (Newton 1). That force multiplied by time integrated over the period of its duration is the impulse or change in momentum it experiences.(Newton 2).
But forces are always paired (Newton 3). So if matter changes its momentum by experiencing forces for a period of time, another body must experience an equal but opposite force for the same period of time and, therefore, an equal and opposite change in momentum.(Newton 3).
So it is not about matter travelling away from the head. It is about matter experiencing a change in motion. The matter ejected from the head experienced a change in its motion due to a force supplied by other matter in the head from which it was ejected. Therefore, that other matter-the remaining head - experienced an equal and opposite change in momentum.
Do you believe that when the bullet fragments exit the head they have a recoil effect on the head?
The laws of physics apply. If the bullet experiences a change of momentum while in contact with matter, that matter experiences an equal and opposite change in momentum. If the matter was the head, the head experiences a momentum change that is equal in magnitude and opposite in direction to the change in momentum of the bullet.
If the bullet experienced an increase in its forward momentum in passing through the head, which seems unlikely, but possible, it would impart some recoil momentum to the head. For example, if the bullet came to a stop in the head and then exploded out of the head due to the built-up pressure in the head and the skull opening up, this would occur.
If the bullet passed through the head and just slowed down, the change in momentum of the bullet is opposite to its direction of travel (ie. toward the shooter). So the head would experience a change in momentum that is equal and opposite to the change in bullet momentum (ie. forward, away from the shooter). In that case, the bullet fragments do not experience any increase in forward momentum on leaving the head so the head does not experience any increase in rearward momentum (recoil). This is, perhaps, the more likely of the two possible scenarios.