Your points are well made.
With 20 turbines, the it seems to me the probability of total turbine failure seems unlikely. I can visualize a turbine problem that developed a control situation that the inexperienced and incompetent staff could not handle. Efforts to solve the problem made at gunpoint exacerbated the problems to the point of unresolvability by other parties than the manufacturer.
We can now encourage further speculative and informed discussion
Venezuela had huge problem with it's electrical infrastructure even before the socialist came to power. I worked with an engineers from there. He talked about a steam turbine that would spin up to 200 rpm with all the stop and control valves closed. All steam turbines have a Stop/Intercept valve that closes immediately when the turbine trips. This valve sits between the turbine and boiler. This should prevent the unit from over speeding. For the turbine to move at all with this valve closed is disastrous. He also mentioned that many of the units he encountered were operated with the voltage regulator in manual mode. If the generator field breaker didn't open at the same time as the generator breaker you would indeed see a large voltage spike on the generator.
I am guessing that when the High Voltage line failed the protective systems started to kick in. These should have automatically closed water flow valves and opened generator breakers. If the water flow valves didn't close in a timely manner it is possible one or more units went to over speed. Thermal units are designed to withstand a ten percent over speed event. I do not know what the max amount is for a hydro turbine generator set. I do know that you can spin one apart if they are not shut down in a timely manner. Secondly one or more units may have suffered a catastrophic voltage spike causing insulation failure.
It could be as simple as nobody remembers how to black start the generating plants. Power plants require electricity to start themselves. Plants that are capable of starting themselves after a shut down with no external power source are said to be black start plants. The problem comes is that black start capabilities are very seldom tested end to end, you have to shut down everything and bring it up from scratch. They may never have even really designed the generating station with a tested black start capability. Also, even if they have the ability the units have likely suffered years of total neglect. They will need time to work around each issue the neglect causes.
One other thing to keep in mind is that all of these protective systems kick in automatically. Generators spin 60 times a second. When power engineers talk about events on the grid they speak in terms of cycles. The two large blackouts that hit the western United States in 1996 or 97 were both over in less than 12 cycles, one fifth of a second. Simply put human beings can't react that fast, at least not to an event that happens maybe once a year. If the operators had bypassed safety systems hoping to catch problems before they led to catastrophe it may have finally caught up with them.
One possibly telling item - nobody in the press, even the government-controlled press has been allowed in to the dam’s turbine halls since the incident. One would expect a socialist government in such straits to want to ‘prove’ that they are working on the issue, even if the turbines would only be props.
Makes you wonder what those turbine halls look like - and if the place has wreckage or redeposited insulation everywhere.