The problem for LB, he is not a good decision maker. Sure he can bring the ball down the court but please don't ask him to execute an offense under pressure. However, what is the worse, if he is forced to play PG he easily loses his confidence in other parts of his game like shooting. Asking LB to play PG is taking away from his best assets.
I did, perhaps too obliquely, address these points in the remainder of my post but I will fill it out. You don't have it quite right - Barbs did not lose his confidence in his shot when he first played point for us, nor did he generally when he played point for Brazil in international competition. It wasn't until he had to back up Steve Nash that he dissolved. I'm not saying he was ever a terrific PG but he was adequate and he shot well. Heck, last year Leandro was the de-facto, if unoffical, backup PG a good bit of the time and his shot didn't disappear, though as YJGS said, he rather sucked at operating D'A's offense.
Now if Porter just handed him the ball and said 'go do what Steve does' - which is what D'Antoni appeared to do when he moved Leandro to the official backup point his first full year as coach - I think Barbs would likely flounder again. This is why my suggestion was to retain set plays for Barbs to run. Also get the other players to buy in to the idea and give the guy lots of support.
Its a rather daring move but Barbs looks like he's on improving track this year on both defense and driving more under control. The team is in desperate circumstances with Nash on track to be totally toast by the playoffs - and yet the team has so much firepower that it has a chance to be a serious contender. Waiting and hoping for Kerr's piddling around to pay off has got to the worst possible plan. I'd rather plunk Dragic in my plan in place of Leandro than bet on Kerr.
Barbs can contribute but on size alone he's not a good fit at SG - but if he could give us 18 decent minutes a game at point we'd be in pretty darn good shape. Bolster his confidence, make sure he knows what he's expected to do, work on the set plays with him, set out the time slots he will be on the floor then trust him to perform. He's capable and he might surprise everyone.