The sentences that contain examples of contested usage are:
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"The postman needs to know who this letter is addressed to so it can be delivered." - This sentence contains contested usage in the phrase "who this letter is addressed to." Critics argue that the correct form should be "to whom this letter is addressed" due to the proper use of grammatical cases.
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"Stephanie started reading the book first, and therefore she finished it before I did." - The phrasing "and therefore" can be debated in terms of its usage; some argue it to be a run-on sentence or that "therefore" should be placed in a different grammatical structure altogether.
The other sentences do not present contested usages.