The grand jury's sole purpose is to determine whether or not the state has grounds to prosecute, not whether or not there were extenuating circumstances or reasonable doubt. So no, the defense will not have the ability to call witnesses, though I do believe they'll have the ability to consult with all witnesses called after the fact.
The fact of the matter is that grand juries usually vote to indict if that's what the prosecution wants. It's even been famously quipped that any prosecutor worth their salt could get a grand jury to indict a ham sandwich if he wanted them to do so. If the state believes that they have grounds for a winnable case, it's near unthinkable that they couldn't get a grand jury their way. Whether or not the state thinks they have a case is what we have yet to know. The prosecutor's intentional pinning it on the grand jury actually leaves me with some questions about what he thinks so far. Maybe just being coy, maybe laying the groundwork to say, "Damn, couldn't proceed with a case, they didn't vote to indict. Shucks."