Parentheses Problem

Recently, I've been encountering a new way to format a parenthetical sentence. And I was following the book's explanation for this new structuring, till I found this (p. 76-77):

• Last year popular fiction accounted for half of all books purchased. (Business and self-help books were the second largest category.)

• Last year popular fiction accounted for half of all books purchased (business and self-help books were the second largest category).

From what I've read, it looks like the period would be on the inside of the parentheses if the sentence was "its own complete" sentence; otherwise, it would be outside. Yet, the only difference between the sentences is format, not content. Does this mean that the definition of "complete" sentences at some point becomes subjective and that subsequently the placement of parenthetical sentences also becomes subjective? Thanks :)

Parentheses vs. em-dashes and commas

The text describes the use of parentheses for including additional information or enclosing asides (p. 149). On p. 151 em-dashes within a sentence are shown to describe “an abrupt change of thought.” The second em-dash example: “Everyone in the class—students and teachers—appreciated the joke” suggests that commas could be used instead because the students and teachers phrase explained the previous statement. Isn’t this including additional information, so technically couldn’t parentheses also be used?

Is there a standard such as using em-dashes within dialogue sentences and using parentheses within text? Is the choice of using parentheses/em-dashes/commas dictated by the editor’s style guide or is one option more preferable? I guess I'm just a little unsure about how the choice of specific punctuation can change the meaning or emphasis of a sentence.

(Sorry if this was already discussed Friday!)