Jent, I have to plead ignorance. I’d be glad to try and verify the story and contact R. Kanievsky and ask if he ever sent out such a letter. But I don’t know how to contact him. If someone would be kind enough to provide me that information, I’ll be glad to send him a letter asking for clarification.
Regarding the mitzvah of abolishing Amalek as applied to finding an Amalekite baby in his crib. I’ve ling been puzzeled by that mitzvah since I’m convinced that were it not for our yearly recounting the story, Amalek would be long forgotten by now. But, there are bigger issues in our religion to bother me. As to the baby in the crib, I too do not know if I have the strength to kill the child, even if I know him to be from Amalek. But that it my weakness and lacking, not the mitzvah. Quite honestly I hope never to come upon the opportunity be tested in this manner.