Because even when Giffey is open, he refuses to shoot?
It's a good question. I think it has to do with Lamb not trusting his judgement. Giffey does a lot of pretty good things when he doesn't have the ball. With the ball in his hands, he's not been terribly good.
I think it has more to do with your first point than with your second.
Giffey hasn't been turnover-prone with the ball in his hands, but it's
because he doesn't try to do anything. He usually catches the ball, holds it in "triple threat" position (remember that?), makes it look like he's going to do something aggressive, but usually passes the ball right back to where it came from.
I think Lamb recognizes that and doesn't want to waste time on the possession. That's why our offense tends to struggle when we either 1) go big (e.g. AO at the 4 and Roscoe at the 3), or 2) have more than one of Giffey, Daniels, or Roscoe in; just not enough threats to do anything with the basketball, and the defense can double or hedge accordingly.