If there were more than one practical joker, then Miller could have been one of them and someone else could have written the note. This possibility undermines the argument, and the author fails to consider it.

We already know that the handwriting is not Miller’s. This is a premise we accept. The exact degree of difference between Miller’s handwriting and the handwriting on the note therefore doesn’t matter.

The author’s conclusion is simply that the joker is not Miller. Why Miller was initially suspected doesn’t relate to whether or not Miller is the joker based on the handwriting sample.

The author doesn’t need to explain why only one piece of evidence was obtained. The author is allowed to make an argument based on whatever evidence is available.

This confuses sufficient and necessary conditions. The author assumes that if the handwriting isn’t Miller’s, then Miller is not the joker. But this doesn’t imply that if the handwriting were Miller’s, it would prove Miller was the joker.