I'm sorry, but you don't seem to understand the passive low pressure emergency core cooling on the BWR 3/4 systems.
As I said numerous times, depressurize the vessel and the passive gravity fed cooling works for up to days. The emergency coolant, that again was available, is located above the vessel. As long as the vessel is depressurized coolant can flow into the vessel at near the same rate as steam is bled off into the spent fuel pools dumping heat. Both the coolant and the fuel pools being used as a heatsink can be replenished with simple external hosing. The depressurization and steam bleed would release some radio gasses, mostly noble gasses ( 135-Xe for the first ~6-8 hours or so, some Kr isotopes ETC ) and Iodine. Heavier elements would tend to stay put inside the vessel.
Yes, its possible had the operators acted differently to mitigate the tsunami damage, the fuel melt may have been prevented. But that is not a cause. There reason the operators did not have the proper instrumentation to deal with a post tsunami wipe-out, is because the plant was not designed to cope with that event.
No, they had all the instrumentation needed. All they really needed was thermometers and volume estimation + maybe a calculator. Instead, being afraid of public outcry over radiation release they chose to trust complex instrument that require precise calibration that had just undergone 4+ minutes of heavy vibrations and then, in parts at least, flooding. See the passive cooling that was available to them, but unused due to fear of public outcry. The plant was capable of coping with the event, it was chosen not to, or at least chosen to cope in the way they had originally thought would result in little to no radiation being released. Unfortunately that was the wrong choice now that we know more from looking back on the disaster.
Had the plant been designed to cope with that event, emergency power sources would have been located in safe areas with protected feeds. Safety equipment would have been located above tsunami levels, and the plant would have had any extra needed instrumentation to perform the necessary operations during that event.
BTW, you can prove that a structure can withstand a force. It is quite easy and common.
The first part of this quote is what I said was lumped into human error, several times already as a matter of fact. And they had all the instrumentation and everything needed to cope, see the above arguments as for why they chose the way they did. As for the second part:
The magnitude of the event was unprecedented, as I said. As to "proving something can withstand a force", that is not what you said. You said ANY force, the plant in question was designed to withstand the biggest tsunami that data said was probable (9m seawalls, plant being located higher than the 9m seawalls, ETC).