The sentence that has a mismatch between subject and modifier is the first one - Though not the prettiest one at the fair, I was sure my cake was the tastiest.
The way this sentence is written, it would seem that the person talking was not the prettiest one at the fair, when this in fact refers to the cake.
Answer:
1The main idea would be "Scientists have lots of evidence to explain why dragons don't exist."
2. Emphasized by the evidence and word choice related to scientific evidence
3. The main idea would be "I have always wondered if dragons existed, and I hope they did."
4. Displayed through the author's nature of questioning and evaluating different kinds of evidence and ideas.
:)