Was Biden's legal team legally required to publicly announce the proposed plea bargain before it was validated by the judge? If so, what is the motivation for that law, and if not, what was their reasoning for doing so?
It seems to me that publicly announcing the proposed deal has a big downside risk and no upside.
If the judge accepts the proposal, then neither side seems to gain anything from announcing it ahead of time. But if the judge rejects the proposal (as in fact ended up happening), then Biden's position seems enormously weakened. If he changes his plea to not guilty and goes to trial, then realistically, most of the jurors will know that he was publicly prepared to plead guilty, which could certainly make them less likely to vote to acquit. (Even if in principle, jurors are instructed to ignore anything about the case that they've learned outside of the trial, and those with extensive knowledge of the case are screened out, in practice that seems unrealistic for a case as high-profile as this one; the information was on the front page of most major American newspapers.)
And even if Biden decides to take another guilty plea, then the mere fact that he is now (arguably) much less likely to be acquitted in court means that the government now has much more leverage to demand whatever terms they want.