In any case, just an excellent piece of writing.
You have apparently looked at the problem a good deal; you believe the "objective" tag really began with the creation of the AP in the 1840's, or did it possibly come later?
1) Remember, ALL newspapers were "partisan" and supported either the Democrats or the Whigs prior to 1850. They were blatant, they were overt, and by one account 80% of GA papers were demonstrably pro-one party or the other. The other 20% were mostly "advertisers" which focused on business (but even then had an agenda).
2) Still, even with the telegraph (which really didn't connect a great deal until the late 1850s), that didn't change newspaper reporting.
3) What changed reporting was the Civil War. It had nothing to do with the "chain of control," or anything like that---it had everything to do with the public's demand for facts as opposed to opinions and rhetoric. Remember, up until 1861, news writing was incredibly flowery and long-winded. The war changed that, and introduced the "inverted pyramid" of writing: most important facts first. What the telegraph did was to further refine that because of the need to economize on words, sort of like Pat Summerall calling a football game: ("to the 40, 30, 20. Touchdown, Dallas.") I have in my book an example of the Confederate "information" officer---the guy in charge of trying to get pro-Confederate stuff into the papers---giving examples of how to economize on words for telegraph transmission and news stories. He elimates all superfluous opinion and extra words.
"Today, forces under Gen. Lee defeated Union army near Five Forks. Union left the field to the north. Losses heavy."
Now, you can quibble with "defeated" (but if the enemy leaves, that's obvious) or "heavy losses," but this would be a typical dispatch. Papers began to publish extensive lists of the names of those killed, without any additional commentary ("died bravely in battle," or some other BS).
By 1865, the large majority of all papers had adopted the inverted style, heavy fact-based reporting, and began to separate "opinion" or editorial pages from so-called "news." While I agree with cIc that this is ultimately an impossible feat, one can come reasonably close by reporting the most obvious, important facts first (recognizing that everyone will not think the same facts are important).
I also agree that a better way to go is a "biased" partisan press so that people can make up their own minds. However, I do think journalism went through a period of about 100 years (1860-1960) where the majority of reporters and editors tried to be "objective" and adopted codes of conduct to try to ensure objectivity (always get more than one witness; get the other side of the story; no unsourced stories; etc.) In other words, I do think for a while most reporters attempted to be "fair and balanced" and "objective." Whether they succeeded is a different question.
One of the problems is that if you ALWAYS get the "other side of the story," it does introduce the notion that there always IS another "side of the story." What was the other side of the story to the Holocaust? Should we "get Hitler's take?" In other words, it legitimizes falsity.
Finally, if you are really interested in this, try to run down an article (you'd probably need interlibrary loan or a college library) from 1973 or 1974 by Robert C. Loewenberg, "'Value-Free' vs. 'Value-Laden' History: A Distinction Without a Difference," The Historian. I can find the exact date if you want it. He shows that attempting to separate "facts" from "values," whether in history or journalism, is impossible, and that "objectivity" was one response---but the other was outright bias against the "status quo," whether it was society, tradition, or whatever. In part, then, that explains the inbred liberalism of reporters.