Soft latch switch circuit with buck converter load does not work
You have drain and source swapped on Q5. The body diode is passing current regardless of the state of the gate.
If you want the circuit on at startup, add a small capacitor (0.1 µF) between the gate of Q5 and ground.
You have a similar problem in your &quot;Autovoltage Selection&quot; circuit. If you apply +12 V to it, both V_out and VBUS will be driven to +12 V, again through the body diode of Q7. This would be very bad for the USB source if you ever connected both simultaneously. Swap drain and source here, too.
If you want a circuit that doesn't oscillate, here's one possibility:

The inputs (pins 2 and 6) are biased to half the supply voltage. When the output is low, a press of the button will pull the trigger pin (pin 2) low, driving the output high. When the output is high, a press will pull the threshold pin (pin 6) high, driving the output low.
Note that in either case described above — and unlike other similar circuits — the output will NOT oscillate slowly if the button is held down. The bias resistors R1 and R2 are sized relative to the output feedback resistor R3 such that capacitor C1 cannot charge to the required switching voltage until the button is released.