1. When you first wrote the manuscript with decedent, you and decedent became joint copyright owners, who could license the work as long as profits were split between you and decedent.
2. When decedent died, her copyright descends to the estate, and then to the estate beneficiary/ies. I assume because you state that partner has legal rights in the manuscript, that partner was given decedent's copyright via a will and probate estate.
3. If all of the above is true, then your rewrite of the manuscript created a derivative work from the original, and that work is 100% yours, but subject to the partner's 50% license obtained through the probate. That is, neither you nor partner can independently license the 2nd manuscript.
4. Any revision of the 2nd manuscript would be subject to your 100% copyright on the derivative, so even if you didn't do any work, the partner could not publish without your consent.
5. There is an alternative legal theory under which your work creates an implied license of your work, granted to partner, whereby you retain your copyright, but partner impliedly obtains the right to edit and distribute the manuscript. In cases where courts have actually found an implied license agreement, the question devolves into exactly what was the reasonable expectations of the parties. Did you grant an unlimited nonexclusive license to revise and republish? Doubtful. I would probably find that you did license the work, but that it was only the work that you wrote, and that by revising it a third time, the license was violated.
The important point here is that a competent publisher is not going to publish without your consent, because there is a risk that you will sue for infringement and win. So, I think you may be overthinking the outcome. You still have a lot of leverage -- though the partner may not recognize it, because she doesn't understand copyright law.
You can send a letter to the publisher stating that there is a substantial dispute over the ownership of the manuscript, and that if it is published or revised without your consent, you will be forced to take legal action to protect your interests.
If you haven't registered the 1st and 2nd revisions of the manuscript with the U.S. Copyright Office, I suggest that you do, so that if you have to sue -- you can.
Note: my analysis identifies a different scenario in paragraph #3, which I did not continue. If "all of the above" is not true, then the outcome here may be different.
Hope this helps.