Sun is back physically, but not mentally. And she's not terminating. As you said, Stivrins isn't back, and may not be back for several more weeks (mid Big Ten schedule). Hames was hurt, and while Evans was a decent fill-in, Cook is rolling with Hames as she recovers from her injury. Orr isn't up to speed after her knee surgery, so she's not a 100% option.
Sun was rumored to be moving to the RS, but for some reason that hasn't happened. Cook has chosen to bench her rather than continue to play her on the left. Which is fine - if she can't or won't move, she's not the best LS hitter and she's going to sit.
Cook has also been experimenting with his lineup all non-conference. I think we didn't play the same lineup in our first six matches. Last night he played a lot of Freshmen, and they struggled, but that's OK.
This year is all about getting the 2021 recruits up to college speed. If they contend for the Big Ten title that's great, but Cook is playing the long game, especially with Stivrins out. These girls will be ready for prime time next year. They're already doing well enough - sweeping a Creighton team that had just swept Kentucky - but there's going to be growing pains as they develop (UNO, Utah & Stanford).