... I’m no expert but .... Wouldn’t the People have to vote for a replacement as they did in the Primarys .... and not have one simply appointed by “The Party?”
No, the DNC can select a replacement.
The DNC can nominate someone else in case of an emergency. That said, I think it is legally too late. Ballots are determined and some states have very early voting.
In 1972 after the primaries and conventions, McGovern’s VP selection, Tom Eagleton from Missouri was found to have undergone mental evaluations; he was replaced by Kennedy in-law Sargent Shriver. The Democratic National Committee made the choice.