All the choices are good. The words aren't always interchangeable. "Slowly" can only be an adverb. "Slower" can also be an adjective and can make comparisons.

I agree that it's not a commonly used phrase. Perhaps someone was trying to be funny. Also, "wrong" is not a gradeable adjective. Overall, this is extremely informal speech. I would recommend that you concentrate on learning standard English first.

That is a very odd choice of words.

Someone make mistakes of weather forecasts on TV or radio.