My "poisoned" larvae recovered a little yesterday after being kept the day before "drowning" in tiepid water for 15 minutes and then let dry on a paper-towel.
Maybe it was a stupid trick but I had wanted to try anyway!
Today they are feeding!

The other larvae seemed to be not affected by the insecticide in
leaves.
Yesterday I provided Nerium oleander branches from another source and washed leaves keeping them in a solution of water + bi-carbonate and then with a solution
0,5% of bleach!
Then I rinsed well the branches and gave to the larvae,
which since yesterday are feeding without warning symptoms.