My final timeline:
Offer accepted - 9/21
Submitted to ROFR - 9/29
Passed ROFR - 10/16
Received closing docs - 12/07
Returned closing docs - 12/17
Notified of closing and deed recorded - 12/29
Received member package (point charts) - 01/13
Received online activation code (email) - 01/25
Obtained Club ID / membership number (by phone) - 01/28
So a day short of 4 months from ROFR submission to account access, and a little over 4 months for the full process. Phew. Glad that's over.
Just to close the loop on this... yesterday we returned from our Disney trip (which, incidentally, included a few nights booked on our new DVC contract). While we were gone, our membership number letter was finally delivered! We left on March 12, so it was delivered somewhere between 11-12 weeks after our contract closed.

The really crazy thing is that the letter was postmarked January 12! It took over two months to get to me. Maybe someone walked the whole way? There was no indication on the envelope that it had been returned to sender, or otherwise misdelivered, and our address was correct. But clearly it got caught up somewhere in the mail system.
Anyway, a couple of takeaways for those who might still be waiting:
- Unless you're prepared to wait indefinitely, go ahead and call after a reasonable wait. Mail issues can and do happen.
- When I called on January 25, I was told that I was "not in the system". That was clearly bollocks, since by then the membership number had long since been mailed, and under normal circumstances I would have already received it. I was also told during this call that it was "normal" to take over a month for accounts to be set up - also untrue, based on my experience and that of others on this thread. So if you've been waiting more than a few weeks and get this sort of answer, don't hesitate to hang up and call back immediately. A different CM might give you a more satisfactory answer.