What the **** is going on with the offense? How can so many guys be struggling at the same time?
Short answer, i think they are worn down. Losing Bryant as long as they did meant they rode Bote pretty hard. You had Heyward down a while as well as Schwarber. Russell played hurt for close to 2 months. Also keep in mind they haven't exactly had it easy in terms of the NL central. They have had someone hot on their ass for months. So, they couldn't exactly rest some guys as much as you'd like to at this point.
Longer answer, you basically have Baez, Rizzo, Zobrist and Bryant playing well the past 30 days. The rest of the line up has been replacement level or in some cases worse than that. They are getting almost nothing out of their OF and Contreras hasn't been the hitter he can be. Caratini's been decent but nothing special. This kind of goes into the conversation we had a few IST's back about chili davis. If you're running a 3 true outcomes offense, HR hitters can carry a team. It's essentially scoring in bunches. Because this years team lacks power either by choice(trying to k less) or by poor performance/whatever other reason, they need contributions throughout the line up. The gist there being you get a hit and keep getting hits to score runs vs like a walk and a HR. However, as mentioned above, when you got 4 guys at the top of your line up who are hitting and another 4 + the pitcher not doing anything you're kinda fucked with the less HR approach.
At the moment the offense is basically hope those 4 guys create offense because if they don't it's going to be a low scoring game. As I said before, I think Heyward coming back really will help matters. The guy puts in good at bats and given they aren't getting anything in the OF that's a big deal. I may dive into Murphy's stats later to see what's going on there because when I've payed attention to his at bats they've been good just with poor outcomes. He saw 21 pitches in 3 at bats the other day and was 0-3. Presumably if you can get Heyward back and he gets going then you could have Rizzo, Murphy, Baez, and Bryant in the infield with 2 of the 3 OF being Zobrist and Heyward. Then you can sorta pick and choose match ups for your 3rd OF and hopefully Contreras turns matters around. If that happens I think the offense is fine.
And while I get that the focus for people is going to be on the offense being shitty right now, don't sleep on the pitching. In the last 30 days the cubs team ERA is 2.98 with a 3.51 FIP. In that time Hamels has a 1.83 ERA(3.66 FIP), Quintana has a 1.84 ERA(2.57 FIP), Lester has a 2.08 ERA(2.46 FIP), and Hendricks has a 1.80 ERA(2.65 FIP). In other words, the starters have been dominant and it's not a luck based thing save for maybe Hamels. The other 3 have great results and the underlying numbers support them pitching that well.