After his assassination, Kotzebue was very much in the minds and hearts of his editors, publishers, and readers, not only from his own original work (numerous plays, and some song-lyrics which were remembered long after his plays were forgotten), but also his editorials, critiques, and essays.
So there was plenty of post-humous publishing going on.

The 21 Juni 1821 date is accurate, in that it was in the issue from that date that the "Merkur" printed the specific essay. I could use the information about when Kotzebue actually wrote that essay.

It would be useful, this information, as Friedrich Förster, who had more important things to do, was rather dismissive of his little Frühlingslied about the Kaiser/König and the Frösche. So much so that Förster never disclosed, that I can find, the actual date in which he wrote the satire. It is already helpful to deduce that his "demagogisch" Frühlingslied was making the rounds of the publishers and editors before Kotzebue's assassination in 1819. I have looked hard, regarding Förster, and it is really difficult to locate documentation of this particular satire before 1820, in fact.

It remains to be noted, what Grishka already knows well.
The assassination of Kotzebue -- after which the killer was convicted and executed -- was most expedient for a certain Prince Metternich. The tragedy gave Metternich an excuse for censorship and even surveillance. Which is quite the ironic turn of events following the publication of "Frühlingsmusikanten."
Metternich in 1819: "Today the greatest evil ... is the press."