Yes, a prosecutor presents evidence to convince a jury to go to trial. They have to influence the jury to agree.

Defense’s part comes at the trial.

The expression “a grand jury could indict a ham sandwich” is a nod to the fact that, often, a grand jury votes in the direction the prosecutor wants them to.

Because they usually bring sufficient evidence, and the jury is only deciding if there is sufficient evidence to move forward. This doesnt decide guilt.

There are plenty of things to complain about when it comes to the US “justice” system. Grand jury decisions aren’t remotely the problematic part.