Update: I received the following reply from NWA TTU:

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When researching the luggage information, I am showing that you may have three pieces of luggage for First Class passengers. The first two bags
can be up to 70 pounds and 62 linear inches and the third bag is 40
pounds and 45 linear inches. Your wife would be able to have the same
amount of luggage since you are traveling together. You may view the
luggage information on our website at nwa.com. You may go to "Travel
Tools" on the home page and then click on "Checked Luggage".

I hope the information that I provided will answer your question
regarding our luggage restrictions for First Class passengers.

Thank you for choosing Northwest Airlines. We value your patronage and
consider it a privilege to serve your travel needs.

Sincerely,

Jan
nwa.com Customer Service

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As you can see, she did not fully answer my questions, as she did not address the GPT-MEM segments being on POS CRJ200s and thus booked Economy Y. I have posted a follow-up TTU in reply to which they will hopefully respond fully and correctly.