The report says the valve status of the isolation return valves was not indicated on the control panel. At 15:50 they lost power to the instruments and no longer knew the reactor water level either. If they know what was done to control the isolation condenser for the rest of the afternoon they're not telling us. However, the statement about not knowing the water level or the valve status could be interpreted as a way to excuse why perhaps the correct action was not taken.
As soon as HPCI was no longer available, the IC was the only thing left to prevent the core of unit 1 from boiling dry.
If there was enough water in the IC to last for only 90 minutes (can you still find the source for that), refilling the IC tank should have been a high priority.
This also reminds me again of the issue of running out of fresh water on site. When they later started pumping highly radioactive water from the flooded basement into the condenser tank (1600 m3) at unit 1, they then wanted to pump water from the condenser to the condenser storage tank (1900 m3), but found it to be full and had to empty that water into the suppression pool surge tanks first.
They also had 10,000 m3 of water previously used in primary cooling system circuits available at the Centralized Radioactive Waste Disposal Facility, which Tepco later dumped into the ocean to make space for highly radioactive water from the basements, saying that the radioactivity of the 10,000 tons equaled that of 10 liters of unit 2 basement water.
It sounds to me like perhaps there was water available that wasn't considered. What kind of a plan did they have?