Why doesn't light destroy the PN junction voltage gradient in a solar cell?
When light strikes the silicon of a solar cell, it produces electron-hole pairs "all over the place", as you put it. But the only ones that matter are the ones that are created within the depletion region of the PN junction. They are the ones that get separated by the E field within the junction to produce the terminal voltage that you can measure (\$V_{oc}\$). The rest just recombine on their own almost right away.

As you say, this is effectively a current source that is in parallel with the diode junction:







simulate this circuit &ndash; Schematic created using CircuitLab

The point is, this current flows all the time. If you don't draw it off through an external circuit, it simply flows through the diode junction itself.

There is no reversal of voltage across the junction. The normal zero-current voltage is merely reduced by the photocurrent until a new equilibrium is reached. It is this difference between the quiescent voltage and the reduced voltage that we can read using an external voltmeter, and the anode is positive with respect to the cathode.