I think Timbuk2 may have what you’re looking for. They have a small bike commuter backpack that looks to have both sweatproof laptop slot and room for clothing and shoes. It’s also got all the straps to keep it stable.
I’ve done well with a camelbak without the bladder, but I run with a tablet, not a laptop. Perhaps some strategic syncing of data and stashing of wardrobe at work might allow you a lighter commuting load