After doing a little more digging into the official instructions for IRS Form 2210, I realized that TurboTax had actually been doing everything correctly (at least as of Feb 21), so the problem was that the IRS was rejecting the return when it shouldn't be. Therefore, even though my return was unchanged by the more recent TurboTax update, I went ahead and refiled it today anyway, hoping that maybe the IRS has fixed things on their end. This time my return was accepted -- in 19 minutes! (YAY!!!) So, with no questionable work-arounds, the IRS is now happy with my return. That's a relief.
