Hope's name has no hidden symbolic meaning. It has blatant, right-out-there symbolic meaning. Hope tells us this herself:
[…] Addie let me change [Hope's name] legally. She made me think hard about what I wanted to be called, got out a book of names with their definitions that we pored through. And when we came to Hope, I knew I'd found it. I think hope is just about the best thing a person can have. (7)
Hope really is hope's guiding principle: things can always get better. People who get to know her think it's a perfect name for her.
How about "Braverman"? Here's a kid who survives abandonment by his father, gives up his college plans to help support his family, and gets himself beat up because he's working so hard to overthrow the corrupt town mayor. Is there a braver man?
One metaphorical use of the word "millstone" is something heavy—a mental or emotional burden (source). Real millstones are very, very heavy, thus the metaphor. Mayor Millstone is just that—a huge financial and morale-destroying burden on the town of Mulhoney. Real millstones grind things down, and that's just what Hizzoner is doing to the good people of the town.
Some of the other names just seem to fit: Gleason Beal, "Slime Scourge of the Earth" (2), does have a sleazy feel to it. Sheriff Greebs sounds a lot like "greed." And Flo—isn't that the required name of every sassy waitress in literature?