Sgt., as far as perimeter defense last year, there's a lot of blame to go around. Our zone was not what it usually is defensively, and there were multiple reasons for that. KJ's defense was one, no doubt. Trevor's defense, while decent overall, lapsed in certain games -- LV comes to mind (Rosiere' penetrated the zone at will). Virginia is another example where our guys, G included, could not check the opposing perimeter players.
So I agree with you that there was a problem. But I don't agree with your argument that it was all KJ's fault and therefore G should be our permanent PG. We can spin around all day arguing about last year. I think you should wait and see how KJ performs this season. I'm expecting improvement on D, in decision-making and confidence with ball-handling and running the pick and roll. If that improvement is not there or KJ's having a bad game, sure .. bring G up top.
But G is not an ACC guard, make no mistake. He's very effective at the 3 and can shoot or get by opposing small forwards reliably. At the point, however, as I said before, he draws a lightening quick ACC guard who will play up on him to stop him from shooting. G can get by some of these guys once in a while, but better ACC guards are difficult for him to drive on. In this scenario, if he can't get by the opposing guard, his only other option is to take a contested shot from deep. Therefore, playing G at PG removes an advantage we have at one spot (3) and creates a problem at another (PG). This is why G as PG is a stop-gap.
You mention that when G plays up top, the 3 "opens up". Well, maybe so, but we don't have anyone with experience to play there. Roberson's skillset (judging from last year) is not a SF (he has no outside game). So Tyler Lydon is the best candidate (since MR is not big enough at 6-5/6-6 to play on our front line). Lydon may be able to contribute at the 3/4 in time, but as good as he looks asking him to walk in and take over starting small forward right off the bat is unrealistic.