Update:

Apparently it does work the way it SHOULD after all; I received the following follow-up response:

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Thank you for contacting nwa.com Customer Service.

Yes, you allowance would go for your entire reservation not just bits
and pieces of it. Therefore, you would be allow to have to 70 pound
luggage on the CRJ planes as well. If you have any other questions
regarding the luggage you can also contact us at 1-800-692-6955 so we
can assist you further if need be.

I trust this information has answered your questions regarding luggage
restrictions.

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

Sincerely,

Samantha
nwa.com Customer Service

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