I was directed to the English translation at the recommendation of @tenlittlebullets and am very glad I found it because Charles Godfrey Leland is a hoot! At first blush and in no particular order, some of my favorite bits are:

The mysterious man on a black pony with a Spanish mustache holding a red-and-black banner with "Liberty or Death" written on it, who somehow didn't manage to make it into any adapatations

Leland refuting Heine's claim that the uprising was spontaneous by citing the aforementioned man and his banner; "such figures are not common at funerals."

Heine taking an opportunity to compare the crowds gathered at hte morgue to the audience of the opera Robert le Diable, which he apparently hated

Frequent invocations of the French Revolution, from the (invented?) figure of the old man dressed "according to the latest fashion of 1793" going to the barricades accompanied by his fretful wife, to Heine's claiming once to have "admired Robespierre, Sanctum Justum and the great Mountain"

"I could never endure being guillotined every day—and nobody did endure it—"

The real thread of admiration for the insurgents, aka "the patriots, who to-day are called rebels"

The comparison to Cleontes and Pantaeus, who are not quite as famous as Orestes and Pylades but that's how they tend to stage the deaths of Enjolras and Grantaire in the musical

"...This was related to me in the Church of Saint-Méry, and I was obliged to lean against the image of Saint Sebastian to prevent my falling to the ground from deep inward emotion, and I wept like a child."

Heine, then 35 years old, helping home a young neighbor who had lost her lover at the barricade and fainted when called upon to identify him at the morgue. He doesn't usually behave so decently and this is a touching change

Heine reports, perhaps unreliably, that the insurgents wore "garlands of willow 'round their small hats," which strikes me as a brilliant excuse to draw Les Amis in flower crowns

A fantastic roast of Lafayette which I will report in full later

Some really chilling descriptions of the "return to order" in the aftermath of the uprising. The brokers at the Bourse cheering as the stocks rise again, shops reopening with broken windows, the quiet progress of arrests, the awkward military review by Louis Philippe, and the state of siege that persisted through June. Nothing, not even the descriptions of the insurgents' deaths, makes me feel worse

"On that day, the eighth of June, so many people thronged ot the morge that there was as long a queue formed as when "Robert le Diable" is to be given at the Opera. I had to wait an hour ere I could enter, and so had time enough to look at that melancholy building, which looks like agreat pile of stones. I do not know what is the meaning of the yellow tablet of wood with a blue centre like a Brazilian cockade, which hangs before the door*. The number of the house is twenty-one. It was sad enough when within to see how anxiously some people looked at the corspes, seeking what they feared to find. There were indeed two agonising scenes. A boy found his dead brother, and stood silent in grief, as if rooted to the spot. A young girl discovered her dead lover, screamed, and swooned. As I knew her, I had the sad task to carry the poor disconsolate creature home. She was employed in a magasin de modes in my neighborhood, where eight young ladies are employed, all of whom are républicaines. Their lovers are all young republicans. I am in this establishment the only Royalist." *I believe that this indicates a hospital, or that the place is under civic sanitary control —Translator.

(Trans. Charles Godfrey Leland)

Robert le Diable is an opera by Giacomo Meyerbeer and Eugène Scribe, which Heine apparently did not like very much, if his poem "Ja freilich du bist mein Ideal" is to be believed. It was a sensation at the time for its thrilling plot, featuring pacts with the devil and a ballet of sinful ghostly nuns.

As for the people Heine encounters — I wonder if anyone has tried to potray them as characters from Les Miserables before, in art or writing. Who could they be: Gavroche and Eponine's brothers? A sibling of one of the other Amis? Muschietta? Bahorel's mysterious laughing mistress, bereft of mirth? There are numerous possibilities.

The speaker begins the poem by perceiving his grief and failing to interpret it, then goes into rapturous detail about the appearance of the river and the rock and the maiden, only to equivocate a little about how the fairy tale ends ("ich glaube..." as a qualifier rather than a confirmation) because he wasn't paying attention to the boatman either. He too only saw the girl.

In the end he understands that the Lorelei "with her singing" has aggrieved not only the doomed boatman but the narrator himself. But it takes the recapitulation of the whole fairy-tale for him to understand what "die Lorelei," both the woman and the poem, have done to him.