Is "petit theft" really only American English and is the phrase usage moving tow...

Merriam-Webster does not connect its petty theft entry with the petit theft one like it does for the adjective, and it says the latter is "US, Law" yet the former contains examples from the U.S.

When was that petit theft usage established in the U.S. and is it only in use in American English; why? Is this usage, and the pronunciation of petit in that phrase (stress on the first or last syllable), consistent in the U.S. for the infraction and if so is something like "In 2020, he was charged with misdemeanor battery and petty theft in Los Angeles" (Jack Brook, Chicago Tribune, 3 June 2026) somewhat inaccurate; or indicative this is actually being replaced with petty for all intents and purposes both in written and spoken forms (except of course in statutes)?

Yes and yes

Petit theft was an English common law crime which bequeathed it to all common law jurisdictions including US ones.

In most jurisdictions, most common law crimes were codified during the 20th century. Outside the USA, the distinction between petit and grand theft was abolished. For example, from New South Wales>

Every larceny, whatever the value of the property stolen, shall be deemed to be of the same nature, and shall be subject to the same incidents in all respects, as grand larceny was before the passing of the Act seventh and eighth George the Fourth, chapter twenty-nine.

So, with the term gone from legal usage, it vanished from general usage.

As for the Anglicisation of “petit” (French for small) to “petty” (English for minor or insignificant), thats due to the way English speakers speak. The French word is pronounced “petty” by most English speakers, and over time gets spelled that way unless the French linkage is strong (e.g. petit four). For example, in California, the crime is petty theft, not petit theft, while in Louisiana, neither term is used in the statute. You can do your own research on the other 48 states, the territories, and Federal law.

Interestingly, the Anglicisation of the feminine “petite” Is less common.