I'm not a lefty so unfortunately I don't have any good tips for running your sling. But if it were my rifle the first thing I would try is sticking a big wad of Sugru over the area between the deflector and the forward assist, and mold it so it flattens the area out, or rounds it out, so it doesn't poke as much.
Beyond that, you can get upper receivers that don't have a forward assist or brass deflector at all. It's more common for pistol-caliber uppers, but they do make them for 5.56 as well. Probably just be conscious of the size of the ejection port so you don't accidentally buy an AR-9 or AR-22 upper.
Also, you can get just full-on left-handed upper receivers as well. They're basically just a mirror opposite of a regular one. These tend to be on the expensive side though because they also need a left-handed bolt and bolt carrier to work.