I think in judging Sudfeld's performance and merits for QB3, the differences between what he's being asked to do now versus what he would be asked to do as an emergency QB has to be acknowlegded.
Here in the preseason, he's being asked to do specific things for specific reasons. They clearly wanted to involve Jamo, they wanted him to run the second-team offense, they probably wanted to look at some other specific players like Cota and Drummond, and all told they wanted him to run probably 20 specific plays.
If we kept him as QB3 and one day have to break the glass for him to enter, he would be asked to do different specific things for different specific reasons. He would likely be running an offense-lite with our first team, behind one of the NFL's best offensive lines, primarily handing the ball off and managing a very simplistic pass game as necessary, just trying to get the ball in the hands of our playmakers.
There might be some overlap between what he's doing now and what he would be doing then, but it's far from a perfect indicator. For instance, both of his interceptions came on deeper passes to Jamo (25-30 yards). They weren't good decisions on his part, but short of an end-of-game situation, he's not likely going to be asked to make those throws as a QB3 entering the game in an emergency situation.