What problem would an open-source version of Carta solve? We use Carta and overall it's pretty good aside from the [very] disturbing problem reported here.

Carta ensures we have a clear record of equity positions in the company and correctly process standard types of transactions like distinguishing between ISO and NSO shares or correctly handle employees leaving that are important for regulatory compliance. We have to delegate certain types of transactions like changes to the employee share pool to our attorney. So it's not just a calculator. If you mess up these things it can expose you to shareholder lawsuits or problems with tax authorities.

That said I'm very interested in the specific details and how Carta resolves this issue. "Rogue employee" has been used as a convenient excuse in the past.

Not disagreeing with this premise, but if Carta were implemented the same way with open source, nothing about Carta being open would have stopped the events from unfolding. It was a cultural and behavioral decision to abuse the platform's trust that way, not a technology one.