I signed a settlement agreement in a case where the opposing counsel was disbarred.
What recourse do I have to waive a settlement reached as a consequence of opposing counsel being disbarred during his overseeing the agreement reached at mediation ?
The only area of law that might remotely apply is CA Code of Procedure 286.
When an attorney dies, or is removed or suspended, or ceases to act as such, a party to an action, for whom he was acting as attorney, must, before any further proceedings are had against him, be required by the adverse party, by written notice, to appoint another attorney, or to appear in person.
If opposing counsel was disbarred before the mediation, it may be argued that you were obligated to inform the opposing side to appoint another attorney, or to appear in person, before any proceedings were to be held against him, and that the case was stayed until you performed this obligation.
However, a mediation is not an adversarial proceeding you were holding against the other party. It is one where you, I presume, amicably came to a resolution with the other side (that I assume appeared, so therefore whether disbarred counsel was there or not is moot), and signed off on such.
That is why a chorus of counsel is suggesting that this line of argument is moot or irrelevant, and that you should focus more on whether you intend to abide by the agreement, breach the agreement, or go into court to actively seek nullification of the agreement, with the latter two options laden with risk that may or may not be appropriate depending on your situation.