What quotes and actions support Squealer as a strong leader?
Squealer isn’t actually a leader at any point in the story. He is a mouthpiece for Napoleon, who becomes the only leader of any consequence once Snowball is run off the farm.
Early in the story Squealer is characterized as a pig who could speak so persuasively that he could “turn black into white.” Throughout the story, Napoleon uses Squealer to smooth over problems verbally to the other farm animals. The point is for the other animals to continue to work at Napoleon’s behest no matter how bad things get. In this sense Squealer represents state-run media, such as the Russian news agency Pravda. State-run media organizations communicate what the government tells them to say, as opposed to a free media, like we have in the United States.
Keep in mind that this story is an allegory. As such, everything represents something else in real life. In the case of Animal Farm, everything symbolizes the Russian Revolution in the early 1900’s.