@wasf, brother I too checked your previous posts and members have guided you rightly. Please flush your radiator using a proper branded flush like LM or Stp. then additionally flush it with plain water until you see clear water coming out. I suspect heavy gunk in your cooling lines from years old cheap coolant which has badly decomposed. Also that gunk was high in the heater core.
This time move to a coolant and cheapest good coolant starts from Zic A premixed coolant. Don't go further low price wise this time.
You oil as you have shown on finger is oxidated and has lost its characteristics I can say that without an oil analysis because its havoline20w50 and you are already 4000km + now. Your rockers, cam shaft and crank bearings are getting a good beating surely due to this dead oil. Change asap.
As far as chances of water leakage into cylinders which is less likely because your car runs cool and as you have reported continuous bubbles coming out for 20 mins, it points out to an extent of gasket damage that must heat up your car and you must feel loss of power. Also using AC would worsen the effect but none of that is happening right? ... so less likely a gasket failure.
Check on your carburetor choke fitting work and get it refit by some good mechanic. secondly the muddy foaming you see in the radiator water could be the remnants of decomposed ancient coolant that had thicken up in the heater cores and now its foaming and colorizing the water in cooling system.
You ignition switch and its wiring needs work as you too have pointed it out and there is no second thought to it, that is where the random engine shutting off issue is coming from...