Looking at the replay a lot of his throws were on the money, it just doesn't seem like the arm strength on the deep shots. He did seem to have a limp so could be a leg injury hampering the deep throws. The 2nd pick was on Pierre, that ball hit both his hands and he just didn't bring it in, instead it bounced around and the Idaho linebacker made a play.
Based off of the last few weeks, I'd say this issues are these:
1) Forcing throws down the field. He did that on an EWU drive for an INT, he did that against Montucky for an INT, he did that against Idaho for an INT. The EWU and Montucky picks were bad in that they were post/seam routes for an easy pick by the safety coming over from the other side. The Idaho pick was a toss up on man coverage on Pierre along the sideline. If Pierre can make a play on the ball we're having a different conversation right now. Which brings me to...
2) Dunniway and the receivers don't seem to be on the same page on the back shoulder/cut off routes. Could be mis-communication with the receiver on an audible pre-snap, or could be a misread during the play. There's been a handful of plays the last couple of weeks in which the throw was to the back shoulder/cut off and there was no one there accept a defender. This mistake resulted in an easy pick against Montucky, and what should have been a pick against Idaho. Something is amiss, but this is an issue that can get rectified.
While as concerning these issues may be, there's no reason to think they can't get ironed out. Marshel is a huge security blanket for this passing attack and only having him in a limited capacity is a major target opposing defenses don't have to account for which also plays into the items listed above. The players just have to execute in their moment and all will be fine.