Populous is the correct word.

"Populous" is an adjective meaning "having a large population." In your sentence it modifies the noun "county."

"Populated" is an inflected form of the transitive verb "populate, which means "to furnish or provide with inhabitants." Using it as an adjective is improper.

Your hyphenation is correct. "Most-populous" is a phrasal adjective; it modifies the noun as if one word.

I don't agree that "pled" is less emotional than "pleaded." I think they carry equal "emotional" weight, if any. Only context can increase or decrease the emotional impact of either, as in your on-his-knees example.

In the context of legal defense, Merriam-Webster defines "plea" as:

(1) : a defendant's answer to a plaintiff's declaration in common-law practice
(2) : an accused person's answer to a charge or indictment in criminal practice

To plead, in this sense, is simply to enter a plea. No begging is implied. Regarding the past tense of "plead," Bryan A. Garner, in his excellent A Dictionary of Modern American Usage, has the following to say:

"Traditionally speaking, 'pleaded' is the best past-tense and past participle form."

Also:

"'Pled,' dating from the 16th century, is nearly obsolete in British English, except as a dialectical word. Nor is it considered quite standard in American English, though it is a common variant in legal usage."

That the word "pled" has fallen out of favor is unlikely part of some “grammar correctness” putsch. It seems far more likely that American news organizations are simply adapting to the global marketplace by adopting the more widely accepted usage.