Hello there! Yes, all of South America is below the Tropic of Cancer. The Tropic of Cancer is located at 23.5 degrees north, which includes part of northern and all of Central America, And all of South America is below that. Here is a map below:
Yes he was, he was justified because article two grants him authority to issue executive order.
Assuming that you're referring to the Unsigned Southern Press review of 1852, i think the writer was using first person point of view plural.
In the review, the writer keep using the word 'we' such in : "we doubt Mrs. stowe would recognize the cogency of this argument"
hope this helps