I added a can of sugar water yesterday to W3, after spraying the dying bees with sugar water. Tonight I found a few thousands bees very active on the feeder, which was already half empty. I opened the hive and inspected each comb. Many bees died in the combs, while there was a good quantity of brood going to hatch very soon. No sign of the queen, the erratic behavior of the survivors tells me she is dead.
I will probably try to merge the survivors with K1, which is very weak.
I had trouble finding sleep tonight after the loss of this colony. It is a sad sight. I better get used to it, with CCD, this is not going to be a rare occurrence. Nevertheless, this loss was entirely my fault.