How was your offer worded? Did it say anything about the decision being forwarded to admissions or anything? I'm not sure if they'd be accountable or not, but I'd think that an email from the department head is pretty solid.
This type of offer situation sounds like one of mine as well. My department head worded it as I was being offered a spot and that the graduate department would contact me with my official offer "soon" but to contact him directly with any questions. The official offer took about 2 weeks because of paper work and having to organize my funding, so his email seemed just like a placeholder so I wouldn't accept an offer to a different program while waiting for the "official" acceptance.