Russell is just all and all bad at the plate right now. He is back to K'ing at a tremendous rate for the past month or so. Schwarber needs to focus going oppo more. He has plenty of power to the opposite field and has the approach to go that way. He is turning on almost everything and the league has noticed. Young kids that will learn, but Joe isn't going to let them learn right now. He is going to put his best team out there to win no matter what the mixture.
Well you know me. I'm not a mechanics guy so I won't get into that debate. But if I had to guess Russell is probably trying to adjust back to the league and obviously that is likely to cause you to k more. With that being said, Russell's 2nd half numbers are still an improvement at .226/.296/.354 with a 8.2%/31.1% bb/k rate vs .248/.303/.426 with a 7.2%/26.6% rate. Yes sept is up to around 35% but it's only around 70 PAs which could easily just be him running into a cold streak or really good pitching. That's not to suggest that is by any means a "good" line for a major league hitter. IIRC MLB OBP average is right around .315 but when he's playing he is at least giving you fairly elite level defense. He's had a 13.3 UZR/150 over 746.0 innings at 2B and an even better 21.5 over 389.1 innings at SS. Assuming we take the lower end of that with the 13.3 that would make him the 5th best SS in the league behind Adeiny Hechavarria(17.8), Andrelton Simmons(16.8), Nick Ahmed(15.7), and J.J. Hardy (14.2).
And when you consider the rather glowing talk on Simmons who essentially thru age 25 has been a .254/.302/.360 hitter, it does add some perspective on Russell at 21 who's hitting .236/.299/.387. If anything, the fact he's been so good defensively only improves my opinion on him because I flat out didn't expect him to hit as a 21 year old skipping AAA mostly and having relatively few(IIRC under 500) PAs above A ball. The fact he's even been up all years suggest to me one of two things. Either the cubs front office felt they literally had no other choice or they value that defense pretty highly. I'd tend to believe given Theo's past trading for Cabrera and stating defense as the reason it's the latter.
Either way, I personally don't see that big of a difference between him and the alternatives. Obviously Castro is on fire now but has been playing 2B anyways. And as for Baez, he's been marginally better than Russell's 2nd half numbers but he's also got a likely unsustainable .405 BABIP. If that comes down to say a more reasonable .350 you're looking more at a .250/.280 hitter. Additionally, while not the "best" sample size, through 262.2 innings at 2B and 291.0 at SS Baez is a -23.6 and -7.4 UZR/150 respectively. That's not to say Baez shouldn't play but as I've long said I feel like he's just a better play at 3B and I feel like his defense has shown there. So, even if I'm likely not the most objective person with regard to Baez it's pretty hard for me to see the logic that you're better off with him over Russell. You're essentially trading a marginally better hitter thus far for a pretty massive downgrade defensively. Now if you want to start Baez over Russell vs lefties I can totally see that logic because Russell's been pretty horrid there and while only having 13 PAs this season, Baez is hitting .417/.462/.583. But overall, I still think Russell provides more down the stretch.