How can I get this PMOS to turn off?
In order to fully shut down the output, you need to bring T8's gate voltage close to its source voltage, and you must make sure this really happens even more with your particular choice of T8 (the NX2301P is rated for low "turn on" gate voltages). T6 needs to be driven into "good" saturation, and looking at the resistors and capacitor at T6's base, this will be hard to achieve.

Note that you must supply current to T6's base (and bring the base-to-emitter voltage well above 0.7 V) in order to turn it on so T8 will turn off, and all this would need to happen while the supply for T6 is around 0.5 V. Try to think of an arrangement where T8 is turned on in an active manner and is turned off using resistors only: Resistor between T8.G and T8.S, active component (transistor) between T8.G and ground.

I don't want to sound discouraging, but I would keep the idea of rearranging R42 as an experiment in thoughts and stop here. Try using a simple and proven concept for building a robust regulator with one pass transistor (instead of a "weak" supply from R24, requiring a soft-start, post-regulator using T8). This different design would, however, be beyond the scope of this particular question/thread.