Hang in there, R.!
This is VERY normal to me, and it sounds like you're doing everything you can to try to help the situation. The new changes in his life are having results right now. No matter how hard we try, as moms, to help our kids through adjustments, having a new baby who needs ALOT of attention, and you going back to work are tough for a 2 year old to adapt to.
Just keep consistent and it should eventually discontinue. He needs ALOT of attention BEFORE he gets into trouble to help detour the problems. I'm not sure how you can do that when you work, but I'm sure the "professionals" at Daycare are qualified to help with this.
I was hit by my older son at this age, and no matter how consistent I was about with him, it still took a few months to stop the behavior.
Let the Daycare handle it their way (as long as it's loving, and fair). They HAVEto remove him from situations that could hurt other children, and I'm sure you understand that. He will get to the point where he will stop hitting/pushing because he will get tired of being "in trouble".....hopefully :o)
Know that most boys go through this, and your is no exeception, and it's not YOUR fault for having this behavior, I'm sure, it's just a part of him figuring out how to get what he wants. And how to get good/bad attention.
Hang in there!