Two months after marching through Boston, half the regiment was dead. (25–26)
The regiment kept on going, despite the huge death toll. It's hard enough to get through the semester, never mind persevering in the face of death. They obviously felt very strongly about what they were fighting for.
Their monument sticks like a fishbone in the city's throat. (29–30)
The memory of these soldiers has persevered for over a century in the middle of Boston, even if it has diminished. It almost seems like an uncomfortable memory for the residents (if you've ever actually had a fish bone stuck in your throat you'd know exactly how uncomfortable that is), like they might want to be rid of it.
He has an angry wrenlike vigilance, a greyhound's gentle tautness; (33–34)
Shaw seems as ready as a wind-up toy. He's a lean, mean, fighting machine, and he's ready to go (which is good considering he was on the march for two months before he died—you'd need that kind of extraordinary energy).
when he leads his black soldiers to death, (39)
The company persevered all the way until the bitter end. Let's face it, this is not a great reward for their perseverance, but it's important to keep in mind that their side (the Union) was victorious, so they didn't die for nothing.
the old white churches hold their air of sparse, sincere rebellion (42–43)
The structures in these old New England towns are trying to hold on to their rebellious identity. They rebelled against England and the church of England, and they are trying to hang on to those roots.
Colonel Shaw is riding on his bubble, (61–62)
Even in this imagined aftermath, Shaw just keeps on tickin'. We can't help but imagine this guy for a second when Lowell mentions the bubble, but we're guessing he means it more figuratively.