That's pretty much what I did.
When I went to the Apple store this morning, they did offer to give me a gift card for the MPB. Seems like it would be easier to just take the MPB, mark on the order that the trade-in had been received and everybody go on with their lives, but whatever. I declined since A.) I'd rather the credit be applied to the M2 balance (I took advantage of the Apple Card 0 percent financing) and B.) The trade-in credit for the MPB is $10 less today than it was in July. And while it's only $10, I don't think it's right I take the hit because Apple can't figure out how to send me a box.
After I posted here, I called Apple support again. After consulting with her supervisor, the rep said I had two options. 1.) Try again to have a trade-in kit sent, but since it failed the first four times, the odds aren't good for it succeeding this time. 2.) Go back to the Apple store and trade in the MPB for the gift card. When I expressed my concerns about the gift card, she said if I call Apple support again they can apply the gift card to the M2 balance. And to compensate me both for the $10 difference and for the trouble I've been through, she offered to send me a pair of Air Pods 2 at no charge. I've already got a pair of Pros, but figure I can use the AP2 at the gym.
So I've been back to the Apple store tonight to trade in the MPB for the gift card. I'll call tomorrow to have the gift card applied to the M2 balance, and hopefully that goes smoothly. The Air Pods are being prepared to ship, and hopefully - unlike the trade-in kit - they actually get sent out.
And just to keep this somewhat on topic, still love my midnight M2. No regrets getting it. I do find it ironic that the whole reason I went the trade-in route for the MPB was because I figured it'd be less of a hassle than trying to sell it. Oh well.