Main question:
Part A - don't know
Part B - don't know
Part C - A guess - Matthew Brady
Bonus question: Well, I found the quote in an article from The Atlantic Monthly. The article is titled "Chiefly about War Matters," but the author seems to have chosen not to reveal his name. So my answer to Part A is going to be based on what the author called himself.
Part A - "A Peaceable Man"
Part B - George McClellan
Edit - Although no players other than myself identified the author exclusively by his pen name, I would have accepted "A Peaceable Man" as a correct answer, since the question did not specify that a correct answer would have to give the author's real name.
hoosier