If Brewer comes in, and he's playing with 4 starters. Regardless if he scored or contributed defensively, his +/- will favor that of the collective starters in that set time. It's a strange stat. I'm not saying that was the case for his numbers for the whole night, but it's one example of many to why +/- is a niche stat, and doesn't tell the story of what the bench players did.
I see your dilemma with giving the 2nd team due diligence. If you wanted to highlight the bench on differentials, you could use the average.
That's just my .02
I am glad to see you are thinking out of the box with your recap. Don't change.