For those following this bizarre situation, I got 2 updates:
1. Website still shows "Interview cancelled" under the N-400 case. No one from Tier 2 has called back yet.
2. Got a letter in the mail re: my I-751 that says "Because we also approved your N-400, you will not receive a new Permanent Resident Card. Instead, once you take the Oath of Allegiance, you will receive a Certificate of Naturalization, which will be proof of your US citizenship."
It's a HUGE relief to at least have in writing that my N-400 is approved, even though it was tied to my I-751 case. Now I guess it's just a matter of waiting the 2-6 weeks until things update on the website.