Among other things, it's very rare for Article 32s to take as long as these have taken. What that means is that more evidence is being examined more closely than is the norm.
If Gen Mattis follows the IO's strong recommendation and dismisses charges against LCpl Sharratt, that would be the rarest of occurances. Murder cases nearly always go to court martials.
Some supporters of Gen Mattis believe he is being a fox. The general does not believe these men are guilty, I've heard it said, but wanted to get everything on record so the Marines could be exonerated. That's just speculation, of course, but it makes some sense given the lengthy nature of the Article 32s.