After trying other ways to address this, I eventually called the TurboTax help line. After a long time on hold, they put me in touch with a CPA, who then spent about another hour trying to fix the issue. His final answer: it is impossible to do this in the online version. Instead, he instructed me to download the desktop version, where the problem is easy to address. (There may be many ways to address it in the desktop version, but the method I used was just to edit form 4952 directly). Boo to TurboTax for having this flaw in the first place and for failing to acknowledge it in the answer to this question; but thumbs up to the CPA who worked so hard to solve the problem!
