Question

1. When using headers, I understand that "Introduction" isn't used because it is assumed that the first paragraph is the introduction. Therefore, is it appropriate to use "conclusion" as a header for the last paragraph?

2. When trying to demonstrate that a sentence was omitted from a quotation, which is correct?
a. [emphasis added]... or
b. ....

Answer 1

To answer your first question, while a heading for the introduction section is not required in APA, a heading for the Conclusion is required.
In answer to your second question, you would demonstrate that part of the sentence from a quotation was missing by using "...this type of structure."
Hope this helps!

Answer 2

1. no
2. a