Nissan Patrol GQ - Heating Issues - I can't find the cause.

[GQtdauto]

Car makers would never sell parts mate that's why but like your thinking and agree totally with what you've said .

[garett]

i had my thermostat seize open. you can get taps for your sump, but i wouldn't. try a cooling system flush like liquid intelligence stuff ( used it it works) dayco / gates made decent belts

[dom14]

Any thoughts about washing the engine out with Hydrochloric Acid ?? - or can you advise a better way of flushing out my engine out,

Yes, many thoughts. Don't do it unless interested in putting a new engine in soon after.
Even avoid chemicals where you can. Reverse flushing with pressure is fine, and a radiator flush chemical from an autopart shop may be necessary depending on how much rust.
Get hold of the repair manual from here and start reading it for the relevant areas so you understand these topics better.
The repair manual's meant for DIY'ers.

Another Question - Why can't I just fit a fixed fan? that always turns,

It's a temperature control/power saving mechanism.

https://en.wikipedia.org/wiki/Fan_clutch

When cold, you don't want the fan to turn at full blow. Electric fans in modern cars are used for that purpose(fans only come on when necessary). Older cars use viscous fan clutch with built in thermostatic trigger.
Fan that turns all the time at full blow is not good, particularly when the engine's cold running. There is less force transferred to the
fan via viscous hub when cold and more force when hot.

- Why are there so many points of failure in a car / truck

That a philosophical question. Bertrand Russel may know the answer to that.

...on second thought, some of them may be intentional to sell parts.

When you thermostat fails - it always fails to a closed positition

No, it doesn't. It can get 'stuck' open as well, or open too much in response to the temperature of the water.
I have two spare thermostats that open too early, stay open too much for a given coolant temperature, effectively
causing the coolant temperature to stay 'too cold' all the time. Not a good thing for the engine, but not as bad as
running too hot. Too cold cause excessive engine internals wear and poor fuel economy.

When the clutch fan / viscous fan fails - it fails to a none working position

More like the silicon "gel" inside the viscous hub loses it's viscosity properties(in relation to temperature) over the years and less and less force is transferred from the water pump axle to the fan via the viscous hub, causing the fan the run slower and slower when the engine's hot and eventually the fan may not run at all, but nobody waits for that point 'cos you'll notice the overheating engine, diagnose and resolve the issue before that. Degeneration of the viscous hub is usually gradual.

Why don't they make a bypass tap so that if your thermostat fails you can manually over ride?

That's idiot proofing I would say. The engine lasts it's natural lifespan when run at the optimal temperature. There always will be an idiot that would use a tap improperly and cause the engine run "too cold" and end the engine's life prematurely. All the thermostats(or most of them) have a small valve on it that opens up when a permanently shut thermostat valve causes the water to trap inside the engine and overheat,. The small valve allows the overheated water/steam to escape but it only helps with buying some time and saving engine block.

Why don't the make an oil tap so it's easy to drain you sump oil? - like my radiator has - so easy

Good idea if you want somebody to sneak under your vehicle and loosen the tap so you can lose oil on the highway and lose the engine. Engine oil is too critical for the health of the engine to allow an easy mechanism to drain the oil.