Update- I FINALLY got my rebate but not without a fight. After numerous calls to the promo dept where I was first told everything looks fine, just wait, then told I submitted the wrong IMEI, then told I again submitted the wrong IMEI after submitting the other one, and after even the promo rep's 'manual override' got denied, I finally got to the bottom of the problem.
Turns out we were supposed to set up activation at checkout, which the original promo's wording did NOT mention(they have since changed it on the new offers to include this important step). Back in Nov I followed the original promo's directions exactly- purchased the T-mobile version, traded in my old phone, activated new phone on my line- but since I didn't activate it at checkout, the rebate system was seeing my new phone as the plain unlocked model hence the repeated "no/ineligible device" denials. I had to contact one of Tmo's higher-ups and explain the story and basically beg for my rebate. His assistant called me a few days later and she was really nice, I gave her all my purchase receipts and whatnot showing I did everything right and she ended up mailing me a physical $150 card.
Maybe this can help everyone else who's having this issue. If you keep getting denied despite being eligible, it is their mistake and they owe you that rebate. Be nice but persistent, and you'll get it. PM me for the higher-up's contact info, it's public knowledge but I feel like I'd be breaking some rule by just posting it here.
I'm glad I finally got it, but honestly if I'd known what a pain in the butt it was going to be I'd have never done the promo at all and just kept my old phone. Zero issues getting my refund from Apple, but I doubt I'll bother with any other carrier rebates in the future.