It is true that the Fifth Amendment right not to be "held to answer for a capital, or otherwise infamous crime, unless on a[n] . . . indictment of a grand jury" is one of several parts of the Bill of Rights the Supreme Court has never held to be incorporated in the Fourteenth Amendment, and apply through it to the states. So I suppose Missouri law might have allowed, or even required, the D.A. to let Wilson testify in his defense to the grand jury. But if that was not the case, by allowing that testimony the D.A. opened to question the grand jury's finding that there was no probable cause to indict Wilson.
The same applies here. Unless Ohio law requires the officer who killed this boy to be allowed to present evidence in his defense to the grand jury, the D.A. should not allow it. That is the traditional rule, and the one that applies in federal cases, as Justice Scalia once explained:
[R]equiring the prosecutor to present exculpatory as well as inculpatory evidence would alter the grand jury's historical role, transforming it from an accusatory to an adjudicatory body.
It is axiomatic that the grand jury sits not to determine guilt or innocence, but to assess whether there is adequate basis for bringing a criminal charge. That has always been so; and to make the assessment it has always been thought sufficient to hear only the prosecutor's side . . . According to the description of an early American court, three years before the Fifth Amendment was ratified, it is the grand jury's function not "to enquire ... upon what foundation [the charge may be] denied," or otherwise to try the suspect's defenses, but only to examine "upon what foundation [the charge] is made" by the prosecutor. As a consequence, neither in this country nor in England has the suspect under investigation by the grand jury ever been thought to have a right to testify or to have exculpatory evidence presented. (emphasis added; internal citations omitted) U.S. v. Williams, 504 U.S. 36, 51-52 (1992)